ax+2y+3a=0 --> a+(2/a)y+3=0
3x+(a-1)y+a+6=0 --> x+[(a-1)/3]y+(a+6)/3=0
直线ax+2y+3a=0和直线3x+(a-1)y+a+6=0平行的充要条件是:
2/a = (a-1)/3 --> (a-3)(a+2)=0
a=3 或 a=-2