Answer:
Length = 15ft
Width = 12ft
Explanation:
First, re-read the problem until you acknowledge it and can put it into your own words. 
This is what I did:
 Find the area of a rectangle by first finding the length and the width. Note that:
L = Length 
W = Width
Find L and W
We will need formula:
 P=2L+2W 
 A=LW
 The length of a rectangle is 3 ft longer than its width :
 L = 3 + W Equation [1]
 "the perimeter of the rectangle is 30 ft" means
 P = 54 Equation [2]
 P = 54 = 2L + 2W 
 54 = 2(3+W) + 2W 
 54 = 6 + 2W + 2W 
 54 = 6 + 4W 
 48 + 4w
  12 ft = w
L = 3 + W 
 L = 3 + 12
  L = 15 ft
Area if needed
A = L*W
 A = (12 ft)*(15 ft)
 A = 180 sq ft
Kavinsky