Answer
$81-90x+25x^2$
Work Step by Step
RECALL:
$(a-b)^2=a^2-2ab+b^2$
Use the formula above with $a=9$ and $b=5x$ to obtain:
$=9^2-2(9)(5x) +(5x)^2
\\=81-90x+25x^2$
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