2x^2+xy-y^2-4x+5y-6
=(2x-y)(x+y)-3(2x-y)+2(x+y)-6
=(2x-y+2)(x+y-3)
a^2-4ab+4b^2-6a+12b+9
=(a-2b)^2-6(a-2b)+9
=(a-2b-3)^2