Question 696765
A=LW
21=(X-2)(X+2)
21=X^2-4
X^2=21+4
X^2=25
X=SQRT25
X=5 ANS. 
L=5-2=3 ANS. FOR THE LENGTH
W=5+2=7 ANS. FOR THE WIDTH.
PROOF:
21=3*7
21=21