To figure this question out, we first need to understand how to solve it. We know that all the angles in a triangle add up to 180 degrees. we can imagine this is a massive triangle. We can do the following
73 + 51 + (21+x) = 180
we can do this, because we already know the other two angles, and we know part of the last angle is 21, and the other part of the last angle is x. We can add them all together.
