Length of the rectangle 
=
15
 feet
Breadth of the rectangle 
=
30
 feet.
Explanation:
Let the breadth of the rectangle 
=
x
So, the length of the rectangle 
=
2
×
x
The perimeter given 
=
90
 feet
As per the formula for perimeter of a rectangle
Perimeter 
=
2
×
 (length + breadth )
90
=
2
×
(
x
+
2
x
)
90
=
2
×
(
3
x
)
90
=
6
x
x
=
90
6
x
=
15
So the length of the rectangle 
=
15
 feet, breadth of the rectangle 
=
30
 feet.