Thevenin Equivalent With complex sources
Just start putting numbers onto your diagram and, because this is homework, I'm not going to feed you the final answer but a halfway answer: -

So, I've given you Ix, can you run with this now? Next step, calculate the voltage across the 7 Ω resistor.
@김현우 did you get your answer as 15 amps?

yes i calculated I_A is 15A :) Thanks! – 김현우