The solution to your problem is as follows:
Given:
(1) 5 + iy = x + i3
In order for complex numbers to be equal the real parts have to be equal and the imaginary parts have to be equal. For (1) we have
(2) 5 = x for the real parts, therefore
(3) x = 5
And for the imaginary parts we have
(4) iy = i3, therefore
(5) y = 3
Answer: x = 5 and y = 3.
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