25a⁴ - 10a²z² - 8z⁴ multiply 25*-8 = -200 to find factors (10*-20 = 200), (10 - 20 = -10)
25a⁴ + 10a²z² - 20a²z² - 8z⁴ factor the left two and right two terms
5a²(5a² + 2z²) -4z²(5a² + 2z²) one factor from each side should be identical
= (5a² - 4z²)(5a² + 2z²)