Answer
The amplitude of the function should be multiplied by $-1$ to reflect its graph about the x-axis.
Work Step by Step
Consider the function $f$, having equation $y=f\left( x \right)$.
When we multiply each of the values of the function by a negative sign, then the graph is inverted vertically.
Let the reflected graph of the given function be $g$ , having equation $y'=g\left( x' \right)$. In order to obtain the reflection of the graph about the x-axis, each value of x will be kept the same while each corresponding value of y will have exactly opposite coordinates, that is,
$\begin{align}
& x'=x \\
& y'=-y \\
\end{align}$
So, it can be written:
$\begin{align}
& y'=-y \\
& g\left( x' \right)=-f\left( x \right) \\
& g\left( x \right)=-f\left( x \right)
\end{align}$
Thus, for reflection about the x-axis, the function value should be multiplied by -1.
