Question 182419
Let
l = length of the field
w = width of the field
A = area of the field


Original Dimensions:
l = 5x
w = 4x
 
New Dimensions : Increasing the length by 12 feet 
l = 5x + 12
w = 4x
A = lw


Substitute the new dimensions l and w:
A = (5x + 12) (4x)
A = 20x^2 + 48
 
Therefore, the new area is 20x^2 + 48x.


Also check http://provingcams.com/ for better presentation of the solution. :)