<u>All three angles of a triangle add up to 180°</u>. In this image, you can see that the angle ∠ACD is 130°. Because ∠ACB and ∠ACD are supplementary angles it means that their angles equal 180° when added together.
If ∠ADC = 130° then:
180° - 130° = 50°
Since we now know that ∠ACB is 50°, we can subtract our two angles from 180° to get the measurement for angle z.