A uniform thin rod AB of length L has linear mass density $\mu(x) =a +\large\frac{bx}{L}$, Where x is measured from A. If the CM of the rod lies at a distance of $ \bigg( \large\frac{7}{12}$$L\bigg)$ from A, then a and b are related as :

Question

$\begin{array}{1 1} a=b \\ a=2b \\ 2a=b \\ 3a=2b \end{array} $