x²+6xy+9y²-16a²+8a-1=(x²+6xy+9y²)-(16a²-8a+1)=(x+3y)²-(4a-1)²=[(x+3y)+(4a-1)][(x+3y)-(4a-1)]=(x+3y+4a-1)(x+3y-4a+1)