You equate the real and imaginary parts of the 2 complex numbers. If you look at
On the left side, x - y is the real part (no i), and on the right side, 5 is the real part. So you set them equal to each other:
Look at the imaginary parts. On the left side you have 2(x + y - 1), and on the right side you have 4. So they're equal:
You can simplify a little bit by dividing both sides by 2:
So now you have the system that Plato wrote above.