Let me explain this the best I can. We're missing point T but we know that S is the mid-point between R and T
The distance across from is -9 -2 = -11 and the distance in height is 4-(-1) = 5
Point T will be the same distance from point A is point R (this is getting complicated)
Since point T will be in the opposite direction as point R you'll have to do +11 and -5 instead of -11 and +5...
So to find T, add 11 across and 5 down to point S(2, -1) and you get:
T = (13, -6)
You can draw it out if it makes more sense!
