I'm new to programming and don't really know what I am doing, so please excuse my inexperience.
What I am trying to do is multiply a number (input by the user) by another number, that is selected based on the users choice of a Drop Down List. For example, if a user chooses A in the Drop Down List, the number (input by the user) B is multiplied by C (a constant associated with the list choice), but if the user chooses D in the Drop Down List, B is multiplied by E (also a constant).
Any help is much appreciated.
I would start with listening for "change" event in your drop down lists - and when such event is handled recalculate your equation.
If you look into documentation for Spark DropDownList there is exactly such example (on the bottom of page) on how to handle "change" event and then recalculate what is displayed on screen: