The key requirement for a parallelogram is that the opposite sides need to have equal length. Therefore,
ON = LM and NM = OL
2x-7 = 4x+3 x+6 = 8y-7
The first equation allows us to solve directly for x, so that's the most efficient place to start. Subtracting 3 and 2x from each side, we get
-10 = 2x
-5 = x
That said, substituting gives a length for ON and LM of -17, which doesn't make a whole lot of sense. I assume you have modified the problem for academic honesty (which I encourage). Hopefully my answer gives you the framework to solve the actual problem in front of you. In a typical problem, both equations that result will involve both x and y. If that were the case, we can first solve one of the equations for one of the variables. So if we take the second equation and subtract 6 from both sides, we get:
x = 8y-13
We could then substitute this into the other equation everywhere 'x' appeared and then solve for 'y'.
If you need any clarification, let me know.