I found an answer from www.quora.com

A and C are **points** in an argand diagram representing the complex ...

A and C already represent **2 points** of the **square** and since they are diagonally ...
So B should be **4**+2i +(-1+**3i**)= 3+**5i** and D should be **4**+2i-(-1+**3i**)=5-1i.

For more information, see A and C are **points** in an argand diagram representing the complex ...

I found an answer from en.wikipedia.org

Complex number - Wikipedia

A complex number is a number that can be expressed in the form a + bi, where a
and b are real ... has no real solution, since the **square** of a real number cannot
be negative. ... to be purely imaginary, and the **points for** these numbers lie on the
vertical axis of the complex plane. ... **For** example, **2** + **3i** is a complex number.

For more information, see Complex number - Wikipedia

If point *A = 2+i ; B = 4+3i ; C = 2+5I ; D = 3i*

A = (2,1) B = (4,3), C = (2,5), D = (0,3)

Using the distance formula \sqrt{({x_{2} - x_{1}}){^2}+({y_{2} - y_{1}}){^2}} between *AB BC, CD, DA*

AB = \sqrt{({4-2}){^2}+({3-1}){^2}} = 4

BC = \sqrt{({2-4}){^2}+({5-3}){^2}} = 4

CD = \sqrt{({0-2}){^2}+({3-5}){^2}} =4

DA = \sqrt{({0-2}){^2}+({3-1}){^2}} = 4