Question 324483
L=W+8
A=LW
84=(W+8)W
84=W^2+8W
W^2+8W-84=0
(W+14)(W-6)=0
W-6=0
W=6 ANS. FOR THE WIDTH.
L=6+8=14 ANS. FOR THE LENGTH. 
PROOF:
84=6*14
84=84