Answer:
The problem seems to be incomplete, after an online search I couldn't find the complete question, to be able to solve the question, I will assume that the known measure is the width W.
For a rectangular prism of length L and width W, the surface area is calculated as:
S = W*L
For the assumption we took, we have:
W = 39.5ft
And we also know that S = 158 ft^2
And we want to find the missing dimension, in this case is L, then we need to solve:
158 ft^2 = W*L
We just need to divide both sides by the known measure, W
We get:
(158 ft^2)/W = L
So you just need to replace the known value, and can find the missing one.