Ans: $b=\dfrac{-6a}{a-1}$
$\begin{array}{rcl}
a(b+7) & = & a+b \\
ab+7a & = & a+b \\
ab – b & = & -6a \\
b(a-1) & = & -6a \\
b & = & \dfrac{-6a}{a-1}
\end{array}$
$\begin{array}{rcl}
a(b+7) & = & a+b \\
ab+7a & = & a+b \\
ab – b & = & -6a \\
b(a-1) & = & -6a \\
b & = & \dfrac{-6a}{a-1}
\end{array}$
