Exercise 4-2

1. Declare and initialize numeric fields. Include price and tax (double), 
     quantity (int). Also declare a double called total, but do not initialize it.
2. Change the message variable to include quantity 
      (example: "Mary Smith wants to purchase 1 Shirt.")
3. Calculate total by multiplying price * quantity * tax.
4. Print a message showing the total cost. 
      (example:  "Total cost with tax is: 25.78.") 