Error undefined reference to C++

ini file main.cpp di folder Sources

#include "hello.h"

int main(){
    cout << "Test procedure Greeting" << endl;
    string name = "Ashleen";
    string id = "1301208111";
    greetings(name,id);
    cout<<"please answer question 1 "<<endl<<endl;
return 0;
}

ini file hello.cpp di folder Sources

#include "hello.h"

void greetings(string x, string y){
    if(y.length() != 10){
        cout <<"Wrong input"<<endl;
    }else{
        if(y.substr(0,2)=="13"){
            cout<<"hello "<<x<<"from School of Computing."<<endl;
            cout<<"you are majoring in ";
            //int z = static_cast<int>(y[3])-48;
            const chart *p=y.substr(3,1).c_str();
            int z = atoi(p);
            switch (z){
                case 1:
                    cout<<"Informatics";
                    break;
                case 2:
                    cout<<"Computing Science";
                    break;
                case 3:
                    cout<<"Information Technolofy";
                    break;
                default:
                    cout<<"..., where again?";
            }
            cout<<endl;
            cout<<"and you'r the ";
            string n= y.sustr(6,4);
            if(n[3]=='1'){
                cout<<n<<"st";
            } else if(n[3]=='2'){
                cout<<n<<"nd";
            } else{
                cout<<n<<"th";
            }
            cout<<" student listed in this major"<<endl;
            cout<<"in "<<"20"+y.substr(4,2)<<endl;
        } else{
            cout<<"hello, you're not from School of Computing, "
            <<"are you?"<<endl;
        }
    }
}

ini file hello.h di folder Headers

#ifndef HELLO_H_INCLUDED
#define HELLO_H_INCLUDED

#include <iostream>
#include <cstdlib>

using namespace std;

void greetings(string x, string y);

#endif // HELLO_H_INCLUDED

dan errornya adalah line 7 di file main  undefined reference to greetings error : Id returned 1 exit status