Usually when we think about points on coordinate planes, we think of them as x-coordinates and y-coordinates. However, when we use function notation, we use f(x) instead of y. (x,y)⇔(x,f(x)) We want to know the value of x when f(x)=4. To do this, we need to find the corresponding x-value of the function when then y-value is 4.
We can see that the function crosses through the point (3,4). Therefore, when f(x)=4, x=3.