As far as I know Cramer's rule only applies to linear systems, which this certainly is not.
However, if you let a=1/x, b=1/y and c=1/z, you will convert it to a system to which Cramer's rule does apply. Then you can solve for a, b and c by Cramer's rule and then go back to the inverses.
If you don't remember Cramer's rule, it tells you how to use determinants to solve linear systems.