Answer
$a)$ $(8,-12)$
$b)$ $(-8,12)$
Work Step by Step
$a)$ Reflection accross the $x$-axis will negate the sign of $y$-coordinate:
$$(8,12)\rightarrow(8,-12)$$
$b)$ Reflection accross the $y$-axis will negate the sign of $x$-coordinate:
$$(8,12)\rightarrow(-8,12)$$
