Answer
$15x^{2}+8x-6$
Work Step by Step
We are asked to subtract $(3x+7)$ from the sum of $(7x^{2}+4x+9)$ and $(8x^{2}+7x-8)$. This is equivalent to the expression $(7x^{2}+4x+9)+(8x^{2}+7x-8)-(3x+7)$.
We can subtract the third term from the first two terms by adding the opposite of the third term to the first two.
$(7x^{2}+4x+9)+(8x^{2}+7x-8)+(-3x-7)$
Next, we can combine like terms.
$(7x^{2}+8x^{2})+(4x+7x-3x)+(9-8-7)=15x^{2}+8x-6$
