[tex]\begin{gathered} 9r+8t=18 \\ r=f(t) \end{gathered}[/tex][tex]\begin{gathered} 9r+8t=18 \\ 9r=18-8t \\ r=2-\frac{8}{9}t \\ f(t)=2-\frac{8}{9}t \end{gathered}[/tex][tex]\begin{gathered} f(-9)=2-\frac{8}{9}\times-9 \\ f(-9)=2+8=10 \end{gathered}[/tex][tex]\begin{gathered} f(t)=34 \\ 2-\frac{8}{9}t=34 \\ -\frac{8}{9}t=32 \\ \text{cross multiply} \\ -8t=288 \\ t=\frac{288}{-8} \\ t=-36 \end{gathered}[/tex]