Use Taylor theorem with Lagrange remainder :
f(a+h) = f(a) + h.f '(a) + h²/2 . f '' (a) + h^3/6 . f ''' (t1), where t1 is between a and a+h ;
f(a-h) = f(a) - h.f '(a) + h²/2 . f '' (a) - h^3/6 . f ''' (t2), where t2 is between a-h and a.
Simplify e(h) with the given formulaes :
e(h) = (h²/12) . ( f '''(t1) + f '''(t2) )
Use triangle inequality :
|e(h)| ≤ h².2M / 12