How Do I Fix This Matlab Code?

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MatLab

MATLAB HELP !!! TAYLOR SERIES?
i have to write a matlab function that accepts the following inputs and yields the following
inputs:
1)f function, inputted as a string 'f'
2)a center of the taylor series
3)n degree of the taylor polynomial
output:
1) P n-th degree taylor polynomial off centered at a


i need the function to output a P nth degree taylor polynomial centered at a
here is the code i have , it works almost perfect but im not sure how to get the derivatives of the functions at "a" with out compromising my (x-a) term...
from what i see the diff(f,k) computes the kth derivative of f, but can not plug
in the a. looks my code will require another matlab function that will do the plugging any suggestions?

function [P] = mytaylor(f,a,n)
f=sym(f);
syms x ;
terms=0;
for k=0:n
fk=diff(f,k);
terms = terms + fk*(x-a)^(k)/factorial(k);
end
P=terms;