So your question is: S is the midpoint of R and T, find T, right?
Suppose T(x,y)
Because S(2,-1) is the midpoint of R(-9,4) and T(x,y) and follow midpoint method we have:
2 = (-9 + x)/2 <=> 4 = -9 +x <=> 13 = x
-1 = (4+y)/2 <=> -2 = 4 + y <=> -6 = y
=> T(13,-6)
