How to invoke function from external .c file in C?

Question

My files are

// main.c  

#include "add.c"

int main(void) {
    int result = add(5,6);
    printf("%d\n", result);
}  

and

// add.c  

int add(int a, int b) {
    return a + b;
}

Answer 1

use #include "ClasseAusiliaria.c" [Dont use angle brackets (< >) ]

and I prefer save file with .h extension in the same Directory/folder.

#include "ClasseAusiliaria.h"