Graphic an equation

[speddy]

Hi I have a little problem. I want to know a value of an equation (give it by the user in time of execution) with differents values of X. I want to graphic function in the screen. I try to find X in the string and replace with the value of X, but when I try to convert (the hall string, with non X, all numbers) to double an error happened. If some one know how to do it please send me an email to: speddy@cvtci.com.ar