This is the problem: $\displaystyle 16x^3y^2-4xy^2$

...and what I tried to do:

$\displaystyle 16x^3y^2-4xy^2$

$\displaystyle 4xy^2\left(4x^2-1\right)$

$\displaystyle 4xy^2\left(4x^2-1^2\right)$

$\displaystyle 4xy^2\left(4x-1\right)\left(4x+1\right)$

But then when trying to solve it in order to check the answer, comes to something different.