Build A Servlet That Does Simple Addition And Subtraction?

I am trying to build a servlet that does simple addition and subtraction then re displays the result on the same screen. I started the building the servlet I am just not sure I understand how to assign the value from a text area to a variable, so that I can complete the logic its still early stages, but its a simple app so it wont be a very long project(I hope lol). My question is how to assign the value that the user inputs to the text area to a variable that I can use?

My code and compile time errors:

C:UsersReignDesktopHTMLBank.java:89: cannot find symbol
symbol : variable Amount
location: class HTMLBank
if(Amount.equals("")||Amount.equals(null))//||Amount.equals(<0))

[Code].....