Question 690477
X-Y=2
X=2+Y

X^2-Y^2=18
(2+Y)^2-Y^2=18
4+4Y+Y^2-Y^2=18
4Y=18-4
4Y=14
Y=14/4
Y=3.5 ANS. 
X-3.5=2
X=2+3.5
X=5.5 ANS.
PROOF:
5.5^2-3.5^2=18
30.25.12.25=18
18=18

5.5+3.5=9 ans. for the sum.