Find i_o using source transforms.

NOTE: Two transforms will be performed in the first step. The elements involved in one transform are shown in purple, the elements involved in the other are shown in green.

In this step, the purple elements will be combined to yield one voltage source and one resistor.

The total resistance is 6 K, thus by Ohm's Law, (watch the sign!)

i_o = - 6 / 6 K = - 1 mA