I cannot get correct roots from this code of newton raphson method

Maybe it is due to not getting a correct derivative of polynomial. Moreover, I cannot convert string to symbolic expression. I get error even if I use sym or str2sym. Also, I want to plot graph of given input expression.

I have been using Matlab R2018a.

[Geoff Hayes]

Jalal - you may want to post the inputs that you are passing into this function. For example, what is your string that represents the objective function? Also, if you are getting an error, please copy and paste the full error to this question.