Greetings from Brasil...
See the attached chart. The midpoint, M, has 8 units down on the Y axis. So the other half will also have 8 units after that point M. This is also true for the X axis.
Just make the difference between points T and M. For M and S they are the same quantities.
(0; 3) and (1; -5)
X: 1 - 0 = 1 (one unit until mid point)
Y: - 5 - 3 = - 8 (8 units until mid point)
To S:
S(W; Z) M = center point coordinate value
W = M + 1 ⇒ W = 1 + 1 = 2
Z = M - 8 ⇒ Z = - 5 - 8 = - 13
S(2; -13)
