To reflect a point over the x-axis you have to reverse the sing of the y-coordinate leaving the y-coordinate of the point the same.
For example if you have the point P with coordinates (x,y) the reflection over the x-axis will be:
Original point → Reflection over x-axis
P(x,y) → P'(x,-y)
So for the point (6,-3) the reflection over the x-axis is:
(6,-3) → (6,-(-3)) = (6,3)