(5y-4x)^2 =

Expand the square using the formula (a-b)^2 = a^2 - 2ab + b^2.

Here, a = 5y and b = 4x.

(5y)^2 - 2(5y)(4x) + (4x)^2 = 25y^2 - 40xy + 16x^2