In this situation, we have two parallel lines that are cut by another line, and the interior angles are the ones created in between the parallel lines and the third line.
If both angles are on the same side, then we must have that the sum of the angles adds up to 180°.
If the angles are A1 and A2 we have:
A1 + A2 = 180°
and, we also know that A1 = 3*A2, we can replace it in the previous equation and get:
