Find $I_o$ in the circuit shown given that $I_1 = 3 mA$, $I_2 = 18 mA$, $R_1 = 4 k\Omega$, $R_2 = 16 k\Omega$, $R_3 = 20 k\Omega$, and $R_4 = 11 k\Omega$.

The equivalent resistance of $R_2$, $R_3$ and $R_4$ between nodes A and B is $R_{234} =$ $k\Omega$
The current through $R_{234}$flowing from node B to A is $I_{234} =$ $mA$
$I_o =$ $mA$