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