we you reflect a point across x-axis you do this
[tex](x,y)\longrightarrow(x,-y)[/tex]
so, the point (5,3) is
[tex](5,3)\longrightarrow(5,-3)[/tex]
to reflect across the x-axis you change the sign of the y coordinate
example
[tex](2,-1)\longrightarrow(2,1)[/tex]
