new problem in eval quest!

hi there guys

I am currently making an app in unity where the user can type a math formula into a text box and it will imply certain behaviour on objects. I'm using the javascript eval to parse the user math expression. My question is this, how can I include a variable into this formula. For example if the code is like this:

Code (csharp):

1. var x = 2;
2. var userExpression = "x*x+x";
3. var y = eval(userExpression);

Thanks in advance