Issue using header file to pull in Functions

Question

New to C++ and trying to use a custom header file for the first time. I'm receiving "undefined reference to" my functions in main() for all of my functions when I compile so i'm assuming i'm missing a piece of important syntax somewhere. I've looked at the docs on other posts and I haven't found an answer or a reference to something different than i'm doing. Code below is just for learning purposes I know there are some better ways to do things. Any thoughts?

Code in Main Source File Named: SixthProgram.cpp

//Write functions that will do the following:
//Get a positive integer value from the user and return that value
//Calculate the area of a circle based on a radius
//Calculate the volume of a sphere based on a radius
//
#include <iostream>
#include <string>
#include "MyMathFunctions.h"
using namespace std;

int main()
{

    string msgFromMain = "Please input a positive integer value: ";

    int integerValueFromUser = getIntegerFromUser(msgFromMain);

    double aCircle = areaOfCircle(integerValueFromUser);

    double vSphere = volOfSphere(integerValueFromUser);

    cout << "The area of a circle that has a radius of " << integerValueFromUser
        << " is " << aCircle     << "." << endl;

    cout << "The volume of a sphere that has a radius of " << integerValueFromUser
        << " is " << vSphere << "." << endl;

    return 0;

}

Functions Header File Named: MyMathFunctions.h

#ifndef MYMATHFUNCTIONS_H_
#define MYMATHFUNCTIONS_H_

const double PI = 3.14159;

int getIntegerFromUser(std::string);
double areaOfCircle(int radius);
double volOfSphere(int r);


#endif

Functions Source File Named: MyMathFunctions.cpp

#include <iostream>
#include <string>
#include <cmath>
#include "MyMathFunctions.h"
using namespace std;

int getIntegerFromUser(string msgFromMain)
{
    //Gets integer value from client and returns value to main
    int integerValueFromUser = 0;
    int loopValidationCounter = 0;

    do
    {
        cout << msgFromMain << endl;
        cin >> integerValueFromUser;

        if(integerValueFromUser <= 0)
        {
            cout << integerValueFromUser << " is not a positive integer value.  Please try again: " << endl;
        }

    }while(integerValueFromUser <= 0);

    return integerValueFromUser;

}

double areaOfCircle(int radius)
{

    return PI * pow(radius,2);

}

double volOfSphere(int r)
{
    return (double)(4/3.0) * PI * pow(r,3);

}