If you can't get "theory" questions, it's because you don't know the basic definitions! Go back and review the definitions. Don't just get a general idea of what they mean, learn them by heart. You use the precise words of definitions in proving theorems and solving problems.
Saying that a complex number has "arg 90" means that the line from the point in the complex plane to 0 make a 90 degree angle with the positive real axis and so it lies on the positive imaginary axis. That is, z+ w is pure imaginary. What does that tell you about its real part?