First of all, you should know that each triangle equals to 180 degrees. So just add 55 + 53 which equals 108. Then you just subtract 108 from 180 (180-108) which equals to 72 degrees.
Therefore, the third missing angle measure is 72 degrees.
(35 + 47 + 42 + x) / 4 = 50 (124 + x) / 4 = 50....multiply both sides by 4, cancelling the 4 on the left side. 124 + x = 50 * 4 124 + x = 200 x = 200 - 124 x = 76 <===