Compilation error when using a header file

[Dylan]

Hello, I hope everyone is having a nice day. i would like to find 
a solution to fix my current problem.
I tried using a header file by linking it with my other .cpp 
files and when I compile it, I get this message:


"Rebuild started at 09:34...
1>------ Rebuild All started: Project: HelloWorld, Configuration: 
Debug x64 ------
1>main.cpp
1>main.obj : error LNK2019: unresolved external symbol "int 
__cdecl add(int,int)" (?add@@YAHHH at Z) referenced in function main
1>C:\Users\Dylan\source\repos\HelloWorld\x64\Debug\HelloWorld.exe 
: fatal error LNK1120: 1 unresolved externals
1>Done building project "HelloWorld.vcxproj" -- FAILED.
========== Rebuild All: 0 succeeded, 1 failed, 0 skipped 
==========
========== Rebuild completed at 09:34 and took 00.838 seconds 
=========="


these are my files:

-----main.cpp-----
#include \<iostream>
#include "add.h"

     int main()
     {
         std::cout \<\< "The sum of 3 and 4 is " \<\< add(3, 4) 
\<\< '\n';
         return 0;
     }
     ------------------


------add.h-----------
int add(int x, int y);
----------------------


-------add.cpp--------
#include "add.h"

     int add(int x, int y)
     {
         return x + y;
     }
     ----------------------

I'm not sure if this information is relevant, but I'm using 
Visual Studio IDE and I haven't made any changes to its settings