When a point is reflected across the x-axis, it's y value is flipped. Here is a formula for your future reference:
X-axis reflection: (x,-y)
Using your given coordinates, you simply plug the numbers in, resulting in final coordinates of (5,3.5).
Because the problem asks for the distance between the two points, you now have to find the difference between -3.5 and 3.5, which is 7.
There you go; the distance between the points is 7 units.