$f''(x) \thickapprox\dfrac{1}{2h^2}[f(x+2h) - 2f(x) + f(x - 2h)]$
I'm supposed to be deriving the above formula and establish an error formula in using them.
This is one of a series of problems like this, and I'm not quite too sure on how to get started on this. (This is in a chapter of Estimating Derivatives and Richardson Extrapolation)
Any tips/pointers?