Ans: C
$\begin{array}{rcl}
x & = & \dfrac{3a}{a+2b} \\
ax + 2bx & = & 3a \\
2bx & = & 3a – ax \\
2bx & = & a(3-x) \\
a & = & \dfrac{2bx}{3-x}
\end{array}$
$\begin{array}{rcl}
x & = & \dfrac{3a}{a+2b} \\
ax + 2bx & = & 3a \\
2bx & = & 3a – ax \\
2bx & = & a(3-x) \\
a & = & \dfrac{2bx}{3-x}
\end{array}$
