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Evaluating polynomials

Evaluating 3x²-8x+7 when x=-2. Created by Sal Khan and Monterey Institute for Technology and Education.

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Video transcript

Evaluate 3x squared minus 8x plus 7 when x is equal to negative 2. So to evaluate this expression when x is equal to negative 2, everywhere that we see an x, we just have to substitute it with a negative 2. So it would be 3 times, instead of x squared, it would be 3 times negative 2 squared minus 8 times-- instead of 8 times x, 8 times negative 2 plus 7. And so what does this give us? So we have 3 times negative 2 squared. Well, that's just positive 4. A negative times a negative is a positive. Minus 8 times negative 2 is negative 16 plus 7. So 3 times 4 is 12. From that, you're subtracting negative 16. Subtracting negative 16 is the same thing as adding positive 16. So it's 12 plus 16 plus 7. And so this is equal to 12 plus 16 is 28. And then if you add 7 to 28, you get 35.