How do i solve the function notation 9x+3y=12 where x is the independent variable

You cannot.You have one equation with two variables,which gives you an infinite solution set.What you have written there is actually the equation of a straight line,which has an infinite set of points lying on it.For any solution,you would need another similar equation,then solving these together would give you the solution which represents where the two lines intersect. 

Thanks for that great answer, Mathcad.......here's a graph of the given line......

https://www.desmos.com/calculator/obvl2gjgys

9x + 3y = 12      =>     y =4 -3x

f(x) = 4 -3y        N( 4/3 ; 0 )