what is f(x)=6x+4.

This is just the graph of a straight line........see it here.........https://www.desmos.com/calculator/gvkkmneqho

\(f(x)=6x+4\)

I have no idea what you want to do.  If you want to subsitute x or a numbrer or expression then I can solve for f(x) or I can isolate x.