Lines f and g are parallel with a transversal forming same side interior angles with measures 3x and 5x + 36.
Theorem: If parallel lines are cut by a transversal, then same side interior angles are supplementary.
The angles measuring 3x and 5x + 36 are supplementary, so their measures add up to 180 degrees
3x + 5x + 36 = 180
8x + 36 = 180
8x = 144
x = 18
The answer is x = 18.