The length of the longer base is 41 ft. 
<u><em>Explanation</em></u>
Lets assume, length of one base is 
 ft. 
As, another base is 19 less than five times the length of this base, so the length of another base 
 
The trapezoid has a height of 18 ft and area of 477 ft²
Formula for Area of trapezoid, 
 , where 
 = Two bases of trapezoid and 
 = height of the trapezoid. 
Given in the question: 
 and 
We have also two bases as: 
 and 
So, according to the above formula...

So, length of one base is 
  and another base 
That means, the length of the longer base is 41 ft.