the angles in a triangle always add up to 180°. to work out the third angle first you have to work out how many ° the angles that you already know add up to. in this case you already have the angles 23° and 35° which add up to the total of 58°. now to work out the third angle you have to subtract the total ° of the other angles from 180 because remember the angles in a triangle always add up to 180°. 180-58= 122 which mean that the third angle is 122°. i hope this helped