# How do you factor (a^2 + 4)^2 - 25a^2 ?

$y = \left({a}^{2} + 4 - 5 a\right) \left({a}^{2} + 4 + 5 a\right) =$
= $\left({a}^{2} - 5 a + 4\right) \left({a}^{2} + 5 a + 4\right)$