Add up the two known angles of a Scalene triangle and subtract them from 180 to find the measure of the third angle. Because all three angles of a triangle have to add up to 180 degrees, subtracting two known angles from 180 will reveal the missing measurement. For example, if you know the measure of angle A is 75 and the measure of angle B is 60, add them together to get 135, then subtract 135 from 180 to find the missing angle measure of 45 degrees.
Use the measurement of supplementary angles to find the measure of the interior angles of a Scalene triangle. Supplementary angles add up to 180 degrees; therefore, if you know the measure of the external angle, you can subtract that measurement from 180 to reveal the measurement of the interior angle of the Scalene triangle. For example, if the external angle measures 130 degrees, subtract that from 180 and find that the measure of the interior angle of the Scalene triangle is 50 degrees.
Measure each angle of the Scalene triangle using a protractor. If you do not know the measure of any of the interior angles or the measure of any of the exterior angles, use a protractor to measure each individual angle. You could also just measure two of the angles, add them up and subtract them from 180 to find the third angle. Just be sure after measuring the three angles that when you add them together they total 180 degrees.