This is an example
Your length of the rectangle is 2 times the width.
Let length = 2x
Let width = x
Now your area = 450m^2
2x.x = 450 ("." means multiply)
2x^2 = 450
Therefore x^2 = 225
Therefore x= 15
If your x=15 , then you know your width.
Length = 2x (2 x 15 = 30)
Therefore:
Length = 30m
Width = 15m
Hope that helped! :)