this should have been posted in the C++ forums

int argc, char *argv[]

is unneeded as you do not use them at all and i don't think your using the cmd prompt, also

var x string;

var is an invalid data type and x and string should be reversed i.e., string x. however since your only using it to store +-*/ using char would be more efficient.

instead of using a loop you can use do... while, and in your switch statement you need to add a break after each statement or it will continue until the end or it reaches a break

#include <iostream>
#include <cstdlib>

using namespace std;

int main()
{
    char x,a;
    double num1,num2;
    do
    {
        system("cls");
        cout << "instructions - type: " << endl;
        cout << "+ to add" << "- to subtract" << endl << "* to multiply" << endl;
        cout << "/ to divide" << endl << "\n\n";
        cin >> x;
        cout << "First Number: ";
        cin >> num1;
        cout <<"Second Number: ";
        cin >> num2;
        switch (x) {
          case '+':
               cout << num1 + num2 << endl;
               break;
          case '-':
               cout << num1 - num2 << endl ;
               break;
          case '*':
               cout << num1 * num2 << endl ;
               break;
          case '/':
               cout << num1 / num2 << endl;
               break;
          default:
               cout << "wrong input!\n";
        }
        cout << "Go again? (y/n) ";
        cin >> a;
    }   while(a=='y');
   return 0;
}