The reflection over the x-axis is given by:
[tex]R(x,y)\to(-x,y)[/tex]And the reflection over the y-axis is given by:
[tex]R(x,y)\to(x,-y)[/tex]Thus, a function that is equivalent to the reflection of ABCD across both axis would be:
[tex]R(x,y)\to(-x,-y)[/tex]