A trapezoid is 64cm^2. The height is 12 cm and one base is 4cm. What is the length of the missing base?

Area (trapezoid) = (1/2) height * ( sum of base lengths)

64 = (1/2) (12) ( 4 + B)

64 = 6 ( 4 + B) divide both sides by 6

64/6 = 4 + B

32/3 = 4 + B

32/3 = 12/3 + B subtract 12/3 from both sides

20/3 cm = B = the missing base length