Answer:
(a,b) ⇒ (-a,-b)
Explanation:
When you reflect over the x-axis, the y-value changes sign.
When you reflect over the y-axis, the x-value changes sign.
So if you reflect across both, both values change sign. That is, if you reflect the point (a,b) across both axes, you will get (-a,-b)
(I've also attached a table that's handy for other reflections if you're curious)