What is the definition of f (x) ?

and where on the graph can you find it?

F (x) is just another way to express the "y" in an equation. f (x) is function notation. f (x) = 3x-4 is just another way of writing y = 3x-4. f (x) is found on a graph as the y value that comes out when the x value is applied to the function.