3ax+21a+4x-12b=5x-3
(3a-1)x=12b-21a-3无解
则x系数为0且常数项不等于0
3a-1=0
a=1/3
12b-21a-3≠0
12b-7-3≠0
b≠5/6
所以a=1/3,b≠5/6