Wow, that was a lot easier than what I was trying to do. I knew it was something easy I had to do as well. The worst bit is that it's my algebra skills that have gone down over the years, over everything else.
Find the equation of the tangent line to the curve y = 2x cos x at the point (pi , -2\pi). The equation of this tangent line can be written in the form y = mx+b where
Now I know I have to plug this into f(x-h) - f(x)/h
or is there a different way to do it?