When you reflect a point across the x-axis, the x-coordinate remains the same, but the y-coordinate is transformed into its opposite, i.e. it changes the sign.
Let's start with the point A(1,2)
The reflection of this point is found by keeping the x-coordinate, and changing the sign of the y-coordinate, that is, the new point A' is
A'(1,-2)
Apply the same with point B:
B=(3,4) -> B'=(3,-4)
Finally, for point C:
C=(5,6) => C'=(5,-6)
