sometimes it helps to draw it..
L-----------M-----------N
(x + 10) 3x
Since M is the midpoint, it splits LN into 2 equal sections....so LM = MN
x + 10 = 3x
10 = 3x - x
10 = 2x
10/2 = x
5 = x
MN = 3x......3(5) = 15 <===
LM = x + 10.....5 + 10 = 15
as u can see, they are both equal