Answer: 42 feet.
Explanation:
 Let's call the shorter side: x
 Based on the information given in the problem, you know that:
 Longer side: 

 Hypotenuse: 

 - Apply the Pythagorean Theorem:
 

 Where a is the hypotenuse and b and c are the legs.
 - Substitute values. Therefore, you obtain:
 

 - Solve for x. Keep on mind that:
 

 Then:
 

 Therefore, the length of the shorter leg is 42 feet.