Question 837574
What you are trying to show as given is this:

{{{f(0)=3}}} and {{{f(4)=9}}}


The way you wrote, "f(0=3, f(4)=9" is faulty and will fail to render, as it did fail to render.


You have two points:  (0,3) and (4,9).
You want the equation for the line defined.
Form, {{{y=mx+b}}}.
{{{m=(9-3)/(4-0)=3/2}}}
{{{b=y-mx}}}
Pick either point.
{{{b=3}}}, which is also OBVIOUS from f(0).
EQUATION: {{{highlight(y=(3/2)x+3)}}}