Answer:
8.49 feet
Explanation:
Characteristics of a rectangle 
-  A rectangle is a quadrilateral 
- It has two parallel sides 
- the diagonals of a rectangle bisect each other and they are equal in length
One diagonal of a rectangle divides a rectangle into two right angled triangles. Thus, the length of the rectangle can be determined using the Pythagoras theorem
The Pythagoras theorem : a² + b² = c² 
where a = length = ? 
b = width = 7 
c = diagonal = 11
7² + a² = 11²
49² + a² = 121²
a² = 121 - 49 
a² = 72
calculate the square root of 72
a = 8.49 feet