Quote:
The problem is on the last line
Nope. The problem is everywhere. You have at least 9 lines of code that are not doing what you are hoping they would do.
1.
The operations on strings are limited mainly to concatenation, with the symbol +.
The result of "1"+"2" is not "3", it is "12"
2.
There is no - operator for strings.
3.
The Convert methods you are trying to use are real functions: they take one parameter, which they can't modify, and they return one value, which you consistently have ignored.
No, Convert.ToDouble(somestring) will not magically turn that somestring into a double; that is totally impossible in typed languages such as C#. You declared firstNum and the others as strings, so that is what they are and will continue to be throughout your Main() method.
4.
And then, your program isn't doing anything useful: it takes some inputs, attempts some calculations, and that is it. Nothing gets printed or output in any other way???
I suggest you stop watching stupid YouTube videos and start reading a good book on C#.