Question 95583
Typically you would solve the equation for the y variable and replace it as f(x) or g(x), etc.

So something like, 
{{{3x+y=9}}}
Could be solved for y.
{{{y=-3x+9}}}
Then you would replace the y with f(x)
{{{f(x)=-3x+9}}}
This way you would solve for y when given an f(x).
So if you wanted to solve for f(2) It would be as follows.
{{{f(2)=-3(2)+9}}}
{{{f(2)=-6+9}}}
{{{f(2)=3}}}
This gives us the solution (2,3)