Simply plug in [tex]\sf{z=15.9}[/tex] into [tex]\sf{\frac{z-6}{3}}[/tex]
So:
[tex]\sf{\frac{15.9-6}{3}}[/tex]
Simplify that more:
[tex]\sf{\frac{9.9}{3}}[/tex]
And that finally becomes
[tex]\sf{3.3}[/tex]
So when [tex]\sf{z=15.9}[/tex] the answer will be [tex]\boxed{\sf{3.3}}[/tex]
