If R is the midpoint of ST, then SR and RT will have the same length.
SR = RT
9x + 3 = 11x - 17
3 = 2x - 17
20 = 2x
10 = x
Now plug that into one of the equations.
SR = 9(10) + 3
SR = 90 + 3
SR = 93
Since you're supposed to find ST, double that number.
93*2 = 186
x = 10
ST = 186
