First we will need a bit more information for i) are you using the forward difference, backward difference, central difference. The central difference approximations are more accurate.

If you are using the central difference then you formula is

This is valid for any j in

Since they didn't give you a point to use fix a j in the interval and calculate

This will give you point you are approximating the derivative at.

Calculate the numerical approximation and then calculate the actual derivative and evaluate it at the point.

The error is the difference between the two values.

Give this a try and post back if you get stuck.