Because the 2 bottom inside angles are identical ( they are both 50 degrees) this means that both sides LM and MN are the same length.
To find the length of MN, set both sides to equal, solve for x, then replace x in the equation.
4x = x+3
Subtract 1x from each side:
3x = 3
Divide both sides by 3:x = 3/3 = 1
Now we know x is equal to 1, we can solve for MN:
MN = x+3 = 1+3 = 4
The answer is D. 4