#### Answer

(a) $x$-axis
(b) $y$-axis

#### Work Step by Step

(a) The graph of $y=-f(x)$ is obtained from the graph of $y=f(x)$ by reflecting in the ___$x$-axis___.
(b) The graph of $y=f(-x)$ is obtained from the graph of $y=f(x)$ by reflecting in the ___$y$-axis___.