1.(x+a)^2-(y-a)^2
=[(x+a)+(y-a)][(x+a)-(y-a)]
=(x+y)(x-y+2a)
2.-9+(x+2)^2
=(x+2)²-9
=（x+2+3)(x+2-3)
=(x+5)(x-1)
3.4-12（x-y)+9(x-y)^2
=[2-3(x-y)]²
=(2-3x+3y)²