Answer
$-x^3+8x^2+2x+7$
Work Step by Step
Let p(x) be the unknown polynomial:
$p(x)-(-x^3+3x^2+2x-1)=5x^2+8$
$p(x)=(-x^3+3x^2+2x-1)+5x^2+8=-x^3+8x^2+2x+7$
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