We need to find the voltage drops across the current source. V = 12W/3A = 4V. Now we can apply the KVL and find the voltage at all the nodes in the circuit (one node, the left one).
We also see that the voltage at the left node is 10V + 12V - 4V = 18V (KVL) Next, we can calculate all the currents in this node. Thus, we end up with this:
And now with the help of KCL, we can find Ix current and Vs voltage value by using KVL.