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