Assuming that the pool is a perfect rectangular prism, then its volume, V, is given by [tex]V=lwh[/tex], where l is the length, w is the width, and h is the height. Manipulate this formula first to solve for length.
[tex]V=lwh[/tex]
Divide both sides of the equation by w*h.
[tex] \dfrac{V}{wh} = l[/tex]
Now, plug in your values for volume, width, and height.
[tex]l= \dfrac{3,600}{(5)(6)} = \dfrac{3600}{30} = \dfrac{(120)(30)}{30} = 120 \ ft[/tex]
The answer is 120 ft.