Since both of the opposite sides are parallel, angle 2 is the exact same as angle 1.
mL2 = 160º
The two unnamed angles are also equal to each other for the same reason.
The horizontal line bisects the two unnamed angles.
There are 180º in every triangle.
mL2 + 2(mL3) = 180º
160º + 2(mL3) = 180º
2(mL3) = 20º
mL3 = 10º