We have to calculate OP.
As N is the midpoint between M and O, and we know that NO = 6, we then can conclude that MN is also 6.
If we add MN and NO, we get the value of MO, that is 6+6=12.
If we substract MO from MP, we should get OP:
MP - MO = OP
14 - 12 = 2
Then, we have that OP = 2.