Answer:
MO= 5x +3
NO= 6x
MN= ( collect light terms) 5x+ 6x = 11x +3
ans= 11x+3
If points {M,N,O} are collinear, and N is between M and O, then this would allow us to use the segment addition postulate like this:
MO = MN + NO
MN = MO - NO
MN = (5x+3) - (6x)
MN = (5x-6x) + 3
MN = -x + 3
Answer: -x+3
