This involves two steps. First, you have to know that all the angles of a triangle add up to 180. So the first thing we do is find the angle of the whole red part. 180 - 73 - 51 = 56. Then, we have that one of the angles near it is 21 and the other angle is x. To find x, we do 51 - 21 = 35. Therefore, 35 = x.
