2x^2+3x+1=0
(2x+1)(x+1)=0
x=-1/2或x=-1
2x^2-7x-15=0
(2x+3)(x-5)=0
x=-3/2或x=5
3x^2-7x-6=0
(3x+2)(x-3)=0
x=-2/3或x=3