$$68/21=x+1/y+1/z$$
$x=\frac{68yz-21y-21z}{21yz}$
$z\neq 0\text{ and }y\neq 0$
$y=-\frac{21z}{21xz-68z+21}$
$z\neq 0\text{ and }x\neq \frac{68}{21}-\frac{1}{z}$