using the two angles in the triangle, 1 and 2, it can be 180- the sum of angles 1 and 2, which is c. using angle 4, it can also be 180 - the equation for angle 4
Choice C says that if you take 180 minus the sum of angles 1 and 2, you will find the measure of angle 3. Choice D says that is you take 180 minus angle 4, you will find the measure of angle 3 due to both angles forming a straight angle.
