Anyone know the answer to number 16. plz

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.