Answer
$9x^4-24x^2y+16y^2$.
Work Step by Step
Use the algebraic identity $(a-b)^2=a^2-2ab+b^2$ where $a=3x^2$ and $b=4y$.
$(3x^2-4y)^2=(3x^2)^2-2(3x^2)(4y)+(4y)^2$
Clear the parentheses.
$=3^2x^2-24x^2y+4^2y^2$
Simplify.
$=9x^4-24x^2y+16y^2$.
