First, you should know that the 3 angles of any triangle add to equal 180°.
So, it's given that one angle is 120° wide and the others are congruent. Since they are congruent, we can assign the same variable to each. Let's say each one equals x. So,
120° + the first unknown angle + the second unknown angle = 180°
120° + x + x = 180°
120° + 2x = 180°
Now we just have to solve for x to find the angle measurement for each of the two missing angles.
2x = 180° - 120°
2x = 60°
x = 60°/2
x = 30°