Calculating spesific value in derivative

hi. i am using the GUI, and i am having an issue about something.

i am getting a function from user, for exp : 2*x+x.^3 and x value from user, for exp: x=2

i want to calculate the value in the derivative of the function, and get the spesific value, for exp: 5

using the 'diff' is not giving me the value

what should i write? thank you

[Daniel kiracofe]

diff should work (assuming you are using the symbolic math toolbox). Could you post your code and the error that you are getting?