Answer:
12
Step-by-step explanation:
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. Note that this is not the end to your problem. They asked for the length of MN, not the value of x. Plugging x into MN give you 4+8, which is 12.
(If you need to show work, here it is)
3x=x+8
2x=8
x=4
MN=x+8
MN=4+8
MN=12
