The function f is defined by ~$f\left ( x \right )=-\frac{1}{x}$~ for all nonzero numbers x. If ~$f\left ( a \right )=-\frac{1}{2}$~ and ~$f\left ( ab \right )=\frac{1}{6}$~, then b =
3
~$\frac{1}{3}$~
~$-\frac{1}{3}$~
-3
-12
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