In that case, you should be able to just call GetText() on one entry to get what the user typed as a string, then use whatever conversion function C# provides to convert it to an int/double/whatever (sorry, not a C# programmer). Repeat with the other entry, and you have your two numbers to do the math with.
Showing the result is similar. If you just want distance traveled (for instance), just add a label to the dialog, and set its Text property to the answer. If you're trying to make a graph, that's a little harder. You'll have to use a DrawingArea for that.