In trapezoid ABCD, sides AB and CD are parallel, angle A = 2(angle D), and angle C = 3(angle B). Find angle B

Since AB is parallel to CD....then BC is a transversal cutting two parallel lines

So.....angles B and C are supplementary [sum to 180° ]

So

B + C = 180

B + 3B = 180

4B = 180

B = 45°