How can i calculate Vs in this circuit knowing Vo=2?
As you found the current flowing through the 2k resistor is 1mA, so it means that the same current flows through the 4k resistor. This helps find the voltage drop across the two resistors. Now you know(I hope so) the voltage across the 6k resistor. Use that to find the current flowing through it. Then use KCL to figure out the current going through 9k resistor. Now that you have found the currents apply KVL to get the unknown voltage Vs. I think I gave you more than enough hints.