Ans: $a=\dfrac{4-5b}{3}$
$\begin{array}{rcl}
\dfrac{4-3a}{b} & = & 5 \\
4-3a & = & 5b \\
4-5b & = & 3a \\
a & = & \dfrac{4-5b}{3}
\end{array}$
$\begin{array}{rcl}
\dfrac{4-3a}{b} & = & 5 \\
4-3a & = & 5b \\
4-5b & = & 3a \\
a & = & \dfrac{4-5b}{3}
\end{array}$
