Backward Difference Approximation of the First Derivative

You are required to write a Matlab script file that finds the backward differntial of f(x) = e(x) at x = 0.15 within an es= 0.1% . You need to find that delta x satisfies the error difference (start with a large delta x, compute the backward difference approximation, calculate ea and check against check for E(de;ta x) )