Since M is the midpoint then QM and MR are equal to each other. So by setting them equal to each other we can find the value of x first.
16 + x = 2(x + 2)
16 + x = 2x + 4
16 = x + 4
12 = x
Now we can substitute in the value of x to find the length if QM.
16 + x
16 + 12
28
Hope this helps :)