x^2-8y^2-2xy-2x+26y-15
=x^2-2xy+y^2-9y^2+18y-9-2x+8y-6
=(x^2-2xy+y^2)-(9y^2-18y+9)-(2x-8y+6)
=(x-y)^2-(3y-3)^2-2(x-4y+3)
=[(x-y)+(3y-3)][(x-y)-(3y-3)]-2(x-4y+3)
=(x+2y-3)(x-4y+3)-2(x-4y+3)
=(x+2y-3-2)(x-4y+3)
=(x+2y-5)(x-4y+3)