Recognize that the volume can be found as follows:
(average height of trapezoid) * (8 ft)(13 ft) = 286 ft^3 total volume.
Then (8 ft)(13 ft) (ave ht of trap) = 286 ft^3.
286 ft^3
Solving for (ave ht of trap): (ave ht) = ------------- = 2.75 ft
104 ft^2
Thus, [ 2 ft + (?) ] / 2 = 2.75 ft.
Remove the fraction by mult. both sides by 2: 2 ft + (?) = 5.5 ft
Subtr. 2 from both sides, we get (?) = 3.5 ft
The missing dimension is 3.5 ft.