The sum of the measures of any triangle will always add up to 180°. Also the sum of two supplementary angles, or angles that are side by side on a straight line, will also add up to 180°. In order to find the measure of ∠B, you need to first find the measure of ∠C. ∠C is supplementary to the angle of 113°, so we can find ∠C: 180 - 113 = 57°. Now that we know ∠A and ∠C, we can solve for ∠B: 180 = 72 + 57 + ∠B or 180 = 129 + ∠B, subtracting 129 from both sides, we get ∠B = 51°.
