The tick marks on the two sides mean those two lengths are the same, which also means the angles opposite them are the same measure. (AKA It's an isosceles triangle.)
Since the two missing angles are equal, we can solve:
28 + x + x = 180
to find that x = 76.
The two angles are 76 degrees.
And since LM = LN, we know LN = 18 in.