To write an expression in function notation, you must solve it for the dependent variable.
Assuming that you want the expression to be written in the form [tex] y = f(x) [/tex] (i.e., assuming that x is the independent variable), we have to solve the expression for y, which means that we want y to remain alone on the left hand side, and move everything else to the right hand side.
So, first of all, we can add 9x to both sides to get
[tex] 3y = 9x+24 [/tex]
Finally, we divide both sides by 3:
[tex] y = 3x + 8 [/tex]
