Find all values of the variable $\displaystyle {x}$ for which the rational expression is undefined.

$\displaystyle \frac{{{8}{x}+{7}}}{{{x}^{{2}}+{1}{x}-{42}}}$

The expression is undefined when $\displaystyle {x}=$

If there is more than one answer, write it as a list, using commas to separate your answers.