Question

Factor 9x^3y-25xy^3 completely.

Answer 1

$9x^3y-25xy^3=xy(9x^2-25y^2)=xy\underbrace{\left[(3x)^2-(5y)^2\right]}_{use\ a^2-b^2=(a-b)(a+b)}\\\\=\boxed{xy(3x-5y)(3x+5y)}$

Answer 2

$9x^3y-25xy^3=\\\\xy(9x^2-25y^2)=\ \ \ |from\ ( a^2-b^2)=(a-b)(a+b)\\\\xy(3x-5y)(3x+5y)$