Answer
$9x^4+4x^3-2x+1$; Degree 4
Work Step by Step
$$(18x^4-2x^3-7x+8)-(9x^4-6x^3-5x+7)$$ $$=(18x^4-9x^4)+(-2x^3-(-6x^3))+(-7x-(-5x))+(8-7)$$ $$=9x^4+(-2x^3+6x^3)+(-7x+5x)+1$$ $$=9x^4+4x^3-2x+1$$
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