Find thevinin voltage
Your proposed solution is wrong because you don't treat a capacitor as open when it is not in DC.

In solving for Vth, you could use any circuit analysis method (e.g. current division, voltage division, nodal analysis, etc). You need only to find the voltage across the 5ohm resistor

Remember, in using Thevenin's theorem, you solve for Rth by turning off all independent sources and getting the equivalent resistance as seen by point a and b (which is the 5 ohm resistor in your case). You've done this part right. In solving for Vth, you simply solve for it without turning off any independent sources. Literally, you just solve for Vth using any circuit analysis method.