Since M is a midpoint of LN, we can see 3x equal to x+8. Remove 1 x to get 2x=8. Solve for x to get x=4. <em>Note that this is not the end to your problem. They asked for the length of MN, not the value of x.</em><em> </em>Plugging x into MN give you 4+8, which is 12.
