Question 350758
The length of a rectangle is five times its width. 
If the area of the rectangle is  405ft, find its perimeter.

l = 2w 

We know the area is 405 so,
lw = w(w) = 2w^2 = 405

5w^(2)=405

Divide each term in the equation by 5.
(5w^(2))/(5)=(405)/(5)

Simplify the left-hand side of the equation by canceling the common factors.
w^(2)=(405)/(5)



Now, The perimeter is the sum of the lengths of all four sides.

2L + 2W= 
18 +90= 108 perimeter

Simplify the right-hand side of the equation by simplifying each term.
w^(2)=81

Take the square root of both sides of the equation to eliminate the exponent on the left-hand side.
w=\~(81)

Pull all perfect square roots out from under the radical.  In this case, remove the 9 because it is a perfect square.
w=\9

First, substitute in the + portion of the \ to find the first solution.
w=9

So, simply, 
 9 ft is the width
and 45 is the length (45 is the length because it is 5 times the width, which is 9)

Proof:

45*9

Multiply 45 by 9 to get 405.
405