we know the independent variable is x, so the function will be f(x)
to find this we need to clear variable Y, so:
[tex]\begin{gathered} 9x+3y=12 \\ 3y=12-9x \\ y=\frac{12}{3}-\frac{9}{3}x \\ y=4-3x \end{gathered}[/tex]So the answer is:
[tex]f(x)=4-3x[/tex]