I am writing a simple program that uses functions found in different .cpp files. All of my prototypes are contained in a header file. I pass some of the functions into other functions and am not sure if I am doing it correctly. The error I get is *"'functionname' cannot be used as a function"*. The function it says cannot be used is the `growthRate` function and the `estimatedPopulation` function. The data comes in through an input function (which I do think is working).

Thanks!

**header file:**

```
#ifndef header_h
#define header_h

#include <iostream>
#include <iomanip>
#include <cstdlib>

using namespace std;

//prototypes
void extern input(int&, float&, float&, int&);
float extern growthRate (float, float);
int extern estimatedPopulation (int, float);
void extern output (int);
void extern myLabel(const char *, const char *);

#endif

```

**growthRate function:**

```
 #include "header.h"

float growthRate (float birthRate, float deathRate, float growthrt)     
{    
    growthrt = ((birthRate) - (deathRate))
    return growthrt;   
}

```

**estimatedPopulation function:**

```
    #include "header.h"

int estimatedPopulation (int currentPopulation, float growthrt)
{
    return ((currentPopulation) + (currentPopulation) * (growthrt / 100);
}

```

**main:**

```
#include "header.h"

int main ()
{
    float birthRate, deathRate, growthRate;
    char response; 
    int currentPopulation, years, estimatedPopulation;

    do //main loop
    {  
        input (currentPopulation, birthRate, deathRate, years);
        growthRate (birthRate, deathRate, growthrt);

        estimatedPopulation (currentPopulation, growthrt);
        output (estimatedPopulation (currentPopulation, growthrt));
        cout << "\n Would you like another population estimation? (y,n) ";
        cin >> response;
    }          
    while (response == 'Y' || response == 'y');

    myLabel ("5-19", "12/09/2010");   

    system ("Pause");

    return 0;
}    

```