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# Finding y intercept given slope & point (old)

An old video where Sal finds the y-intercept of the line y=17/13x+b given that the line passes through (5,8). Created by Sal Khan.

## Want to join the conversation?

• Where do you get the "24" from?
• I'm going to assume you mean his multiplication at . All he's doing is breaking up the operation 8*13 into groups to make it easier to do in his head. Given the distributive property:
``a(b+c) = ab + ac thus 8*13 = 8(10+3) = 8*10 + 8*3``

8 times 10 is 80, and 8 times 3 is 24. Add them together and you get 104. It's more steps than doing 8 times 13, but since they're on the times tables and well practiced/memorized, it's easier to do that way!
• still didn't make sense how did u get 104
• MHarmon,
8- 85/13 To subtract the fraction, you must first find a common denominator.
8/1 and 85/13 has a least common denominator of 13.
To change 8/1 to something over 13, you multiply by 13/13
(8/1)*(13/13) - 85/13 And multiply the numerators and denominators
(8*13)(1*13) - 85/13 And not 8*13 is 104 (which is where the 104 came from.
104/13 - 85/13 Now you can abstract the numerators
(104-85) / 13 =
19/13

I hope that helps!
• Is there any other way to get the y-intercept?
• In the Video, could you please show this work on a graph? To show the answer y-intercept b?
• What made him go from 8 = 85/13 + b to 8 - 85/13 = b, how come it was a subtraction symbol and not an addition symbol?
(1 vote)
• He moved 85/13 to the left side of the equation in order to get b alone. So he had to subtract it from both sides (because it's originally being added/positive).

So it becomes 8 - 85/13 = b
• Is there a video where I can figure out how to solve a problem like: Write the equation and give the y-intercept for the line that passes through the given points. (One example would be (4,2), (-3,9)
(1 vote)
• How did he get he number 8?
(1 vote)
• The problem statement gave Sal the point (5, 8).
This means x=5 and y=8
So, the 8 was part of the info given for the problem.
• Line AB has a slope of −4/5. If A (1,4) and B (x,12) , what is the value of x?
Please do not flag this, I know its a homework question but i looked everywhere to find the solution to these kinds of problems. I even asked my teacher, she tried to explain it but i didn't understand, however I told her I'll figure it out since other students had questions too but now obviously I cant figure it out so any help would be appreciated.
(1 vote)
• You can the method described in the 1st response. Or, you can also use the slope formula: m = (y2-y1) /(x2-x1)
-4/5 = (12-4)/(x-1)
Simplify: -4/5 = 8/(x-1)
You now have a proportion. Cross multiply to get: -4(x-1) = 5(8)
Then, solve for X.
• How many points can find out, because I am stuck on that, I know y=mx+b is y-intercept and slope.
b=-1 and I want to know which can I graph out to find my slope?
(1 vote)
• To find the slope, you need a 2nd point you know is on the line. If you have the graph, then find any point where it looks like the coordinates are integers (not fractions or decimals). If you don't have a graph, what information were you given besides the value of b=-1?
• I'm doing an assignment for my 8th-grade math teacher and I cannot find the coordinates.
Following numbers: 0,3 2,11 4,19 6,27, and 8,35. Can you help me please?
(1 vote)

## Video transcript

The following line passes through the point 5 comma 8, and the equation of the line is y is equal to 17/13x plus b. What is the value of the y-intercept b? So we know that this point, this x and y value must satisfy this equation, so we know that when x is equal to 5, y is equal to 8. So we can say-- so when x is equal to 5, y is equal to 8. So we can say 8 must be equal to 17/13 times x times 5 plus b, and then we can solve for b. So if we simplify this a little bit, we get 8 is equal to-- let's see, 5 times 17 is 50, plus 35 is 85-- is 85/13 plus b. Then to solve for b, we just subtract 85/13 from both sides. So we have 8 minus 85/13. 85/13 Is equal to b. And now we just have to subtract these two numbers. So 8 is the same thing as-- let's see. 80 plus 24 is 104, so it is 104/13-- this is the same thing as 8-- minus 85/13, which is going to be-- let's see. This is 19/13. Is that right? Yes, if this was 105, then it would be 20/13. So this is 19/13, so it's equal to 19/13, which is equal to b. So the equation of this line is going to be y is equal to 17/13x plus 19/13.