C :: Creating Simple Calculator By Splitting The Program In 3 Files

I am new to C programming, I have been given an assignment to create a simple calculator by splitting the program in 3 files. It should have 2 .c files and 1 .h... I went through the internet extensively and could only come up with this.

main.c:

Code:

//Calculator main.c
#include <stdio.h>
#include <conio.h>
#include "Functions.h"
int main() {
float x = 0, y = 0;
int operation;

[Code]...

Functions.c

Code:

#include "Functions.h"
extern float x, y;
float addition (float a, float b) {
return a + b;

[Code]...

Functions.h

Code:

#ifndef FUNCTIONS_H_INCLUDED
#define FUNCTIONS_H_INCLUDED
float Sum(float a, float b);
float difference (float a, float b);
float remainder (float a, float b);
float product (float a, float b);
#endif

When I do a 'cl main.c' on the Developer Command window for VS2013, i get an error that reads :

main.obj
main.obj : error LNK2019: unresolved external symbol _difference referenced in function _main
main.obj : error LNK2019: unresolved external symbol _product referenced in function _main
main.obj : error LNK2019: unresolved external symbol _addition referenced in function _main
main.exe : fatal error LNK1120: 3 unresolved externals