Question 745820
The farmer has 42 ft. of fence. In the problem, he arranges it into a rectangle. 
P (perimeter) = 42ft. 
L (length) = 2x
W (width) = x 
Recall that the perimeter of a rectangle is P = 2 (L+W)
Subsitute now the P, L and W. 
The new equation is 42 = 2 (2x + x)
Use the distributive property. Therefore it will become 42 = 4x + 2x 
Find now the x. 
4x + 2x = 42
6x = 42
x = 7

Answer:
L = 2(7) = 14ft. 
W = x = 7ft. 

Checking:
P = 2 (14 + 7)
   = 2 (21)
   = 42ft. 


--There you go! :D