Question 696759
X^2=21+4X
X^2-4X-21=0
(X-7)(X+3)=0
X-7=0
X=7 ANS.
X+3=0
X=-3 ANS. 
PROOF:
7^2=21+4*7
49=21+28
49=49
AND.
-3^2=21+4*-3
9=21-12
9=9