Differentials and mean value theorem? Please help!

Using the formula f(x) = f(x0)+(x-x0)f′(x0)+(1/2)(x-x0)f′′(cx)

(valid for a function f continuous
on a closed interval containing x0 and x and twice differentiable in the corresponding open interval)
for some cx between x and x0, estimate the error in the above differential approximation.