A uniform thin rod AB of length $L$ has linear mass density $\mu ( x ) = a + \frac { b x } { L }$, where $x$ is measured from A. If the CM of the rod lies at a distance of $\left( \frac { 7 } { 12 } L \right)$ from A, then $a$ and $b$ are related as:\\
(1) $2 a = b$\\
(2) $a = 2 b$\\
(3) $a = b$\\
(4) $3 a = 2 b$