What does the state me f(3)=18 mean

Hey there! I'm happy to help!

The normal function notation is f(x)=something. If you switch the x, the x is the function switches. If your function was f(x)=2x+1 and you wanted to plug in an x value of 2, you would write f(2)=2(2)+1 which is also f(2)=5.

So, in your case, this means that if you input 3 into the function, your result will be 18.

Have a wonderful day! :D