So for this equation, all you have to do is multiply by the reciprocal of [tex]- \frac{1}{84} [/tex] , aka -84, on each side and it'll look like this:
[tex](-\frac{84}{1}) * (-\frac{6}{7}) = -\frac{x}{84} *(-\frac{84}{1}) \\ \\ \frac{504}{7} = \frac{72}{1}=72=x[/tex]
tl;dr: The correct option is C.
