7x²-63
=7(x²-9)
=7（x-3）（x+3）
a³-a
=a(a²-1)
=a(a+1)(a-1)
3a²-3b²
=3(a²-b²)
=3（a-b）（a+b）
y²-9（x+y）²
=(y-3(x+y))(y+3(x+y))
=(-3x-2y)(3x+4y)