The answer is 34 2/3. To find this you first find the area of the base which is (length x width) divided by 2 which in this case equals 21, then you multiply the base area by the height, 21 x 5, which equals 105, and last you divide that by 3 or multipy by 1/3, so 105/3 equals 34.999 which is can be written in fraction form as 34 2/3
Hope this helps, also sorry if my explanation is confusing