[tex] \frac{x}{12} = \frac{y}{13} = \frac{z}{15} =k\\
=\ \textgreater \ \frac{x}{12} =k=\ \textgreater \ x=12k\\
=\ \textgreater \ \frac{y}{13} =k=\ \textgreater \ y=13k\\
=\ \textgreater \ \frac{z}{15}=k=\ \textgreater \ z=15k\\\\
3*x+5y-z=3*12k+5*13k-15k=36k+65k-15k=\bold{86k}\\\\
[/tex]
