Answer
$y$
Work Step by Step
A reflection about the $y$-axis of a point $(x,y)$ yields the image as: $ (−x,y)$.
When we replace $x$ by $−x$ in the original function $f(x)$, then we get a reflection about the $y$-axis of the function $y=f(x)$.
Thus, the missing word in the givens statement is $\bf{y}$.
