Ans: $a=\dfrac{2b-5}{3b}$
$\begin{array}{rcl}
\dfrac{5+b}{1-a} & = & 3b \\
5+b & = & 3b-3ab \\
3ab & = & 2b-5 \\
a & = & \dfrac{2b-5}{2b}
\end{array}$
$\begin{array}{rcl}
\dfrac{5+b}{1-a} & = & 3b \\
5+b & = & 3b-3ab \\
3ab & = & 2b-5 \\
a & = & \dfrac{2b-5}{2b}
\end{array}$
