We are given that,
The S is between R and T.
I am going to explain it visually.
As you can see, S is in the mid of R and T. So by figure/definition
RT = SR + ST
Now, put the values in the equation
100 = (2x+7) + (x - 6)
100 = 2x + x + 7 - 6
100 = 3x + 1
100 - 1 = 3x
3x = 99
x = 99/3 = 33
Now, put the value of 'x' is ST equation
ST = x - 6
ST = 33 - 6
ST = 27
Hence, the value of ST is 27.