First, we need to know what the formula really is. Is it
which is what you wrote, or is it
With Z1= 4+ j10, Z2= 12- j3, Z1+ Z2= 16+ j7. Now, write the three numbers, 4+j10, 12- j3, and 16+ j7 in polar form: for 4+ j10, and [itex]\theta= arctan(10/4)= arctan(5/2)= 1.19.
Multiplying and gives