What exactly do you need help on? To use superposition you just look at each source's contribution one at a time while disabling the others (short voltage sources, open current sources).
Edit: Sorry, didn't see the 3 Ohm resistor in there. Just an example of applying superposition is the contribution from the 15V supply; When the 10V is shorted and the 3A is opened, the 15V flows through 5 ohms and 6 ohms in series and then into a parallel combination of 3 and 7 ohms. Therefore we have 15/(6+5+(7//3)) for the 15V contribution to Iy.