f(x) is basically just y, but it is also telling you what the value of x is. So if f(4) then that means that x is 4, so you can just replace x with 4 and simplify. Hope this helps :)
The graph of y = 2x+1 will never go through the point (2,3) because the graph is a linear function which means it goes in a straight line. Knowing this, when y = 3, x = 5 this means that the function will never hit (2,3)
