All angles in a triangle must add up to 180 degrees so if two angles 63 and 90 add up to 153 then the last number added to 153 (x+153)=180 so 27 degrees
It's basic subtraction. you subtract 7 1/8 with 4 4/8 by turning one of the one from the seven to 8/8 and add that to 1/8 so you get 9/8. Then 6 9/8 - 4 4/8 = 2 5/8.