The clue is in the first sentence. MO bisects LMN. This means that LMN is split into two equal halves: LMO and NMO.
LMO = NMO
7x - 29 = 2x + 31
7x - 29 + 29 = 2x + 31 + 29
7x = 2x + 60
7x - 2x = 2x - 2x + 60
5x = 60
5x/5 = 60/5
x = 12
We now know the value of x, and can solve for anything that involves x. We need to find LMN.
LMN = LMO + NMO
LMN = 7x - 29 + 2x + 31
LMN = 9x + 2
LMN = 9(12) + 2
LMN = 108 + 2
LMN = 110