Answer
$8x^2+x$
Work Step by Step
$(9x^2-3x)+(4x-x^2)
\\=9x^2-3x+4x-x^2$
Combine like terms to obtain:
$=(9x^2-x^2)+(-3x+4x)
\\=(9-1)x^2+(-3+4)x
\\=8x^2+x$
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