On the complex plane, the number [tex] a+bi [/tex] is mapped onto the point with coordinates [tex] (a,b) [/tex].
In other words, the x coordinate is the real part of the number, while the y coordinate is the complex part of the number.
Viceversa, if you start from a point [tex] (x,y) [/tex], you can identify the number [tex]x + iy [/tex].
So, the endpoints of the diagonal are the points [tex] (1,2) [/tex] and [tex] (-2,-1) [/tex]. These are points A and C in the attached figure.
This means that points B and D have coordinates
[tex] B = (-2,2),\quad D = (1,-1) [/tex]
So, the correspondant complex numbers are
[tex] B = (-2,2)\mapsto -2+2i,\quad D = (1,-1)\mapsto 1-i [/tex]
