Answer
$\mathrm{See\:explanation\:below.}$
Work Step by Step
A function, according to the definition, must pass the vertical line test.
If a curve is symmetric with respect to the $\:\mathrm{x-axis}\:$, then there exists points $\:(x_1,y_1)\:$ and $\:(x_1,y_2)\:$ such that $\:y_1=-y_2.$
Therefore, if we take a vertical line $\:x=x_1\:$, it would intersect the curve in two points. This must not occur if the graph is a function.
