Answer
$25x^2-90x+81$
Work Step by Step
A binomial squared is equal to the square of the first term plus or minus twice the product of both terms plus the square of the second term.
$(a - b)^2= a^2 - 2ab + b^2$
$(5x-9)^2=(5x)^2-2(5x)(9)+9^2=25x^2-90x+81$