-(x+2y)²+25(x-2y)²
=25(x-2y)²-(x+2y)²
=[5(x-2y)+(x+2y)][5(x-2y)-(x+2y)]
=[5x-10y+x+2y][5x-10y-x-2y]
=(6x-8y)(4x-12y)
=8(3x-4y)(x-3y)