Because angle 4 = 65°, we know that the angle on the other side on 4 (with no number on it) must equal 180° when added to 65°. This is because they are supplementary angles, meaning that they add up to 180°. So if you do 180°-65°, you should get 115°, which is the measure of that angle.
Since the lines are parallel, that means that angle 1 and the angle outside of 4 must be equal, which means that angle 1 = 115°.
