4x^2-(x-y)^2=(2x-x+y)(2x+x-y)=(x+y)(3x-y)
25(x+y)^2-81(x-y)^2=(25x+25y+9x-9y)(25x+25y-9x+9y)=(34x+17y)(17x+34y)=4(2x+y)(x+2y)
4x^2-(x-y)^2=(2x-x+y)(2x+x-y)=(x+y)(3x-y)
25(x+y)^2-81(x-y)^2=(25x+25y+9x-9y)(25x+25y-9x+9y)=(34x+17y)(17x+34y)=4(2x+y)(x+2y)