I just looked it up and this is what I found:
L=5+W
LW=84
(5+W)W = 84
W^2 + 5W -84 = 0
either factor, complete the square or use the quadratic formula
factoring is trial and error, look for factors of 84: 4,3 and 7
(W+12)(W-7) = 0
W= 7,-12, ignore the negative width
L=7+5 = 12 = length
7 inches by 12 inches
7x12 =84 square inches
