All the information you need is given to you. You just need to plug it all in.
You are given your two x values (5 & 6) now, to find f(5) & f(6) just plug them into the function and find your y values.
f(x) = y
When you see f(x) that means that you are solving for y in terms of x
If you are given an x value and a function f(x), you can plug the x value into every single x in the function and it will calculate the y value associated with that x value. So f(5) = -80 See if you can go from there!