Intro to C++, NEED HELP!!!

I have to write a program to calculate the diameter, the circumference, and the area of a circle with a radius of 6.75. Please tell me what I am doing wrong...I am going on 5 hours, and I am lost:-( Any help would be greatly appreciated.

//************************************************

//Calculations of a circle program

//This program finds the diameter, circumference

//and the area of a circle with a preset radius.

//************************************************

#include <iostream>

#include <iomanip>

#include <cmath>

using namespace std;

float radius ;

float pi ;

int main()

{

radius = 6.75; //Size of radius

pi = 3.14159; //Size of PI

cout << "The radius of a circle is : "<< radius << endl;

diameter = 2*radius;

circumference = diameter * pi;

area = pi * radius * radius;

cout << "The diameter is " << diameter << endl;

cout << "The circumference is " << circumference << endl;

cout << "The area is " << area << endl;

return 0;

}