OK, without knowing your skill level with C#, it's going to be difficult to answer this for you without actually writing all the code. The first parts are easy - basically, you have started an instance of calc, but you are storing the result in a local variable. If you want to end that same process, you should make the variable a member instead, and use that to end the application. To close the process, you would typically want to do this:
_process.CloseMainWindow();
_process.Close();
Now, as to actually reading the value of the textbox, that's where the code becomes a whole lot more complicated because you are going to have to jump into the world of the Windows API to manage this.
To be honest, a simpler method would be to simply roll your own calculator application and use that - there's a pretty good article
here[
^] that might help.