Not all code paths return a variable

Hi Everybody! I new to coding with C# in unity. I have however learned c++, and am learning java. I got the book Learning C# by developing games with unity. And I am stumped.
The code I am supposed to write down is an image, so I can’t copy and paste it. Here is the image I have:

[18543-c#+code.png|18543]

This is my code:

using UnityEngine;
using System.Collections;

public class LearningScript : MonoBehaviour 
{
	int number1 = 2;
	int number2 = 3;
	int number3 = 7;


	// Use this for initialization
	void Start () 
	{
		int answer = 
		AddTwoNumbers(number1, number2) +
		AddTwoNumbers(number1, number3);
			
		
			DisplayResult(answer);
	}
	
	// Update is called once per frame
	void Update () 
	{

	}
	
	int AddTwoNumbers(int firstNumber, int secondNumber)
	{
		int result = firstNumber + secondNumber;
		Debug.Log(result);
	}
	void DisplayResult(int total)
	{
		Debug.Log("The grand total is: " + total);
	}
}

You need to return the result. Insert the following between lines 28 and 29 (and it is in your book):

return result;