If point A is the midpoint then that means line segments XA and AY are equivalent so you can set:
3x=5x-6
(now you solve for x)
2x=6
(divide by 2 on both sides)
x=3
So BASICALLY,
XA= 3x= 3(3) = 9
AY= 5x-6= 5(3)-6 =9
So now you know the length of both sides, add those together and you get
XY=18