Hi Amber,
A circle has to have 360 degrees in total to be a complete circle.
3 angles: 110 + 110 + 110 = 330
That means the 3 angles alone is not a complete circle.
Now, the question states what is the missing angle or the 4th angle.
360 - 330 = 30
Answer: The missing angle will be 30 degrees to complete the circle.
Hope This Helps :)