System not declared in scope?

I know its simple code, How do I fix "System not declared in scope" problem?

#include<iostream>
using namespace std;

int main(void)
{
    system ( "TITLE Calculator" );
    system ( "COLOR 2" );
    char cChar;
    double dfirstnumber;
    double dsecondnumber;
    char cDoagain;

    do
    {
        system("CLS");
        cout << "Please enter the first number you would like to use."<< endl;
        cin >> dfirstnumber;
        cout<< "Please enter the operation you would like to perform." << " (+,-,*,or /)" << endl;
        cin >> cChar;
        cout<< "Please enter the second number you would like to use." << endl;
        cin >> dsecondnumber;

        switch (cChar)
        {
            case '+':
                cout << "The answer is: " << dfirstnumber << "+" << dsecondnumber << "=" <<
                (dfirstnumber + dsecondnumber) << endl;
                break;
            case '-':
                cout << "The answer is: " << dfirstnumber << "-" << dsecondnumber << "=" <<
                (dfirstnumber - dsecondnumber) << endl;
                break;
            case '*':
                cout << "The answer is: " << dfirstnumber << "*" << dsecondnumber << "=" <<
                (dfirstnumber * dsecondnumber) << endl;
                break;
            case 'x':
                cout << "The answer is: " << dfirstnumber << "x" << dsecondnumber << "=" <<
                (dfirstnumber * dsecondnumber) << endl;
                break;
            case 'X':
                cout << "The answer is: " << dfirstnumber << "X" << dsecondnumber << "=" <<
                (dfirstnumber * dsecondnumber) << endl;
                break;
            case '/':
                if(dsecond