Question 659970
Starting with 
25x^4 - 100y^2
(5x^2 - 10y)(5x^2 + 10y) mixed signs due to - 100y^2
25(x^2-2y)(x^2+2y)

Alternatively,
factor out 25 immediately
25(x^4-4y^2)
25(x^2+2y)(x^2-2y)