To be a parallelogram, both pairs of opposite sides must be equal in length, i.e. LM=ON and LO=MN
5y-7=x-9 (sub x=4)
opposite sides are congruent...did you make a diagram ? do it now !
You have BAD information in your problem !
ON = LM
8x - 8 = 7x - 4
solving for x gives: x = 4
However, if x = 4, then NM (which you say is (x - 9)) would be NEGATIVE...
CAN NOT HAVE A NEGATIVE DIMENSION !
Check your problem, check your post...repost the problem correctly !