if (num2 < 0) die("The ans cannot be nagative"); this would only be true if num2 > num1.
As far as your problem 1) 'Max' would be a string not a character. So you will have to find a new name for it maybe 'M.' As for fixing this simply replace the output (cout) statements with ans = variable;
If you do it that way or the way smac did you will have to create a special case where you don't output this: cout << num1 << " " << op << " " << num2 << " = " << ans <<endl; if that is the problem you are referring to. A simple if statement should do the trick.