What have you tried?
It can be useful to use the definition of a logarithm, for example:
If $\displaystyle x=a^{(1-\log_a(z))^{-1}}$ and you have to evaluate:
$\displaystyle \log_a(x)$
Substitute the given in the equation and so you get:
$\displaystyle \log_a\left(a^{(1-\log_a(z))^{-1}}\right)=(1-\log_a(z))^{-1}$
Try to reform and substitute ...