I've done some research already and discovered that the formula I need to do this is
With $a$ and $b$ being the start and end points of the section of curve I want to get the average of.
What I need to know is how I translate a maths $y = ???$ (featuring $x$) function into $f(x)\;dx$, so I can actually work out what the formula I need is.
I gather that $f(x)$ means "a formula of a curve" and that $f(x)\;dx$ means a formula for figuring out the area beneath the curve of the given $f(x)$, but I haven't been able to find a manual for how to translate a given $f(x)$ into the $f(x)\;dx$ that gives it's area.
Can somebody help me with this?