解（1）3ax-b=0 (a不等于0)
3ax=b
x=b/3a
解（2）3-mx=nx+4 (m+n不等于0)
(m+n)x=-1
x=-1/(m+n)
解（3）(a-2)x-a-2=0 (a不等于0)
(a-2)x=a+2
x=(a+2)/(a-2)