Answer :

TSO
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]
(z-6) turns to (15.9-6) which is 9.9 then divide that by 3 and you get 3.3