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The volume of a trapezoidal prism follows the rule for regular prisms. V=Bh where V is the volume, B is the area of the base of the prism(in this case a trapezoid) and h is the height of the prism. The area of a trapezoid(in this case the base of the prism) is .5(b1+b2). It is called the base but it is just the top and bottom of the prism not the sides. b1 and b2 are the lengths of the two parallel lines of the trapezoidal face on the prism. Plugging in for B gives us the equation V=.5(b1+b2)h
Once you locate on the prism the trapezoidal face look at the lengths of the two parallel sides and plug those values in for b1 and b2. Then look how long the prism is and plug this value in for the height. Then just evaluate for V by doing all the indicated operations.
Please let me know if you need any clarification. I'm always happy to answer your questions.