十字相乘法
(1)
(x-2)(x+1)=0
x=2或x=-1
(2)
(x-3)(x+1)=0
x=3或x=-1
(3)
(x-6)(x+1)=0
x=6或x=-1
(4)
(x+3)(x+1)=0
x=-3或x=-1