-say in the situation "C" was "X"; angle ∠C and ∠D are supplementary angles so ∠D-180=∠C
-all triangle, angles sums =180° so ∠A+∠B+∠C=180
-if the triangle were to be a right triangle you could use 30,60,90 or 45,45,90 but you need to know the measure of one of 2 of the angles