This is a right angle, meaning that its measures add up to 90 degrees. You can tell this is a right angle by the box in the left corner of the angle. We know that the left side of the angle has a measure of 77 degrees. So to find the remaining angle measure, we subtract 90 from 77. We do this because 90 is the total angle measure, and 77 is one of the angles’ measures. So 90-77 is 13. So your answer would be 13 degrees. Hope that helped :)