How to call a function from another file into main function in c?

I am using qt and created a c project. I have created two files one header file and source file. I have declare a function into header file. So that I can call it from main function. But when I compile and run, I got "undefined reference to " error. How to solve this issue ? I am using qt 5.5 ide.

My Code:

header file chapter_1.h

#ifndef CHAPTER_1_H
#define CHAPTER_1_H

//include all header files
#include <stdio.h>
#include <stdlib.h>
#include <conio.h>

/* function declaration */
int sum(int x, int y);

#endif // CHAPTER_1_H

source file

//include header files

#include "chapter_1.h"

int sum(int x, int y)
{
    int result = x+y;

    return result;
}

main file:

#include "chapter_1.h" 

int main()
{
    sum(23, 23);

    return 0;
}