abs{(z-i) /(z+i)}

I think they want you to write it terms of x and y (where z = x + iy), unless they tell you what numerical value z is supposed to take.

And if that's all they want, then this is fairly easy. Remember that | z1 / z2 | = | z1 | / | z2 |.

first get the complex value out of the numerator.

BTW the answer is +1

Assuming z is purely real? Then yes.