There is a theorem that says if parallel lines are cut by a transversal, then same-side interior angles are supplementary (add up to 180 degrees).
If lines p and r were parallel, then angles 28 and (3x + 2) would add up to 180 degrees.
(28) + (3x + 2) = 180
3x + 30 = 180
3x = 150
x = 50
