Hello I really need help here:

3. Differentiation.
In this problem, you will derive an expression for computing the second derivative at a point using the function values at 3 points: .

Write the Newton form of the interpolating polynomial that goes through these three points

Differentiate the polynomial twice to find a approximation for f''(x)

3 Write an expression for the error in using this approximation

Ok for 1 and 2 but i'm stuck at 3 can someone help me?
Thank you everyone