Since opposite angles are supplementary in a trapezoid we can formulate the following equations to find angles C and D.
Since C and B are supplementary (opposite angles), then the sum of both angles would be equal to 180.
m∠C = 180° - m∠B
m∠C = 180° - 31°
m∠C = 149 °
Since A and D are supplementary (opposite angles), then the sum of both angles would be equal to 180.
m∠D = 180° - m∠A
m∠D = 180° - 62°
m∠D = 118 °