Hey there! I'm happy to help!
A function is kind of like a machine you put a number into and then it changes it and gives you a new number. This function is called f, and x is the number you put into it. So, if we have f(5), this means that 5 is our input that the function will change.
We see that f(x)=x+4. This means that f(5)=5+4, which is just equal to 9.
Therefore, f(5)=9.
Have a wonderful day! :D