Read through the introduction so that you understand how we arrive at the formula
\[
x_{\text {new }}=x_{\text {old }}-\frac{f\left(x_{\text {old }}\right)}{f^{\prime}\left(x_{\text {old }}\right)}
\]
This formula is applied repetitively, starting with an initial value of \( x \). However, Newton's method will fail for a poor initial choice of \( x \).
If our function is \( f(x)=x^{3}-27 x \), what initial values of \( x_{\text {old }} \) would cause division by zero in this formula? Enter your answer as a comma separated list of values.
\[
x_{\text {old }}=
\]
