we have that
a*(3x-8)=b-18x
3ax-8a=18x+b
The only way for this to happen (non-trivially) is if the coefficients of x are equal, and the constant terms are equal
so
3ax=18x-----> 3a=18--------> a=6
-8a=b---------> b=-8*6-------> b=-48
the answer is
a=6
b=-48