Answer
$81-90x+25x^2$
Note: standard form lists variables first in decreasing order of exponent, so this answer could also be written as: $25x^2-90x+81$.
Work Step by Step
$$(9-5x)^2$$ $$=(9-5x)(9-5x)$$ $$=(9\times 9)+(9\times (-5x))+(-5x\times 9)+(-5x\times (-5x))$$ $$=81-45x-45x+25x^2$$ $$=81-90x+25x^2$$
Note: standard form lists variables first in decreasing order of exponent, so this answer could also be written as: $$25x^2-90x+81$$
