Question

25(x-y)^2 - 4(x+y)^2

Answer 1

25(x-y)^2-4(x+y)^2=
[5(x-y)-2(x+y)][5(x-y)+2(x+y)]=
(5x-5y-2x-2y)(5x-5y+2x+2y)=
(3x-7y)(7x-3y)