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 \(f'\), we can see that \(f'\) 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 ：\(\boxed{x=-6, x=-2, x=2, x=6}\)