Can anyone teach me how to solve this problem

We have constructed two angles with angle measures, 30° and 50° respectively.
Recall the angle sum of a triangle is 180°; the formula for angle sum of any n-sided polygon is formulated by: 180(n - 2), where n is the number of sides.

Since we need the three angles to equal to 180, we can call the third angle x.
Hence, our equation becomes:
 
x + 30 + 50 = 180
x + 80 = 180
x = 100

Thus, our third angle is 100°