We know that the sum of all 3 angles in a triangle is 180°
Two angles are given to be 77° and 62°
We can find the 3rd angle by: 77 + 62 + x = 180 ( x is the angle to be found, you can use any variable, if not given) 180 - ( 77 + 62 ) = x 180 - 138 = x 41°