The graph of $f^{\prime }$, the derivative of $f$, is shown below. Determine the values of $x$, if any, at which the function $f$ has a relative extrema on the interval $(-9,9)$.

Step 1 ：The relative extrema of a function occur where its derivative is either 0 or undefined, and the sign of the derivative changes from positive to negative (indicating a local maximum) or from negative to positive (indicating a local minimum).

Step 2 ：From the graph of $f^{\prime }$, we can see that $f^{\prime }$ changes from positive to negative at $x=-6$ and $x=2$, and from negative to positive at $x=-2$ and $x=6$.

Step 3 ：Therefore, the function $f$ has relative extrema at these points.

Step 4 ：So, the values of $x$ at which the function $f$ has a relative extrema on the interval $(-9,9)$ are $x=-6$, $x=-2$, $x=2$, and $x=6$.

Step 5 ：$\boxed{{\displaystyle x=-6,x=-2,x=2,x=6}}$