What is a positive value of x for which 9^(-cos x) = 1/3?
NOTE: The (-cos x) is the exponent in this case.
I'll nudge you in the right direction:
$\displaystyle 9^{-\cos(x)}=\frac{1}{3}$
Note that $\displaystyle \frac{1}{3}=3^{-1}$ and $\displaystyle 9=3^2$
So we can rewrite the equation as this: $\displaystyle 3^{-2\cos(x)}=3^{-1}$
I'm sure you can take it from here.
--Chris