Class to class function calls

Question

How do I call a function that is in one class from another
class? My first class is creating instances of a second class. The
second class draws itself. I want it so that if a user clicks on an
instance of the second class, the first class will know which
instance of the second class was clicked. I try to use a
clickHandler inside the second class that then calls the function
(doThis) in the first class, but I get this error:
1180: Call to a possibly undefined method doThis.
Here is a very simplified version of what I have going on:

Answer 1

right. yet Engine is the Document Class for the FLA. there is
only one instance of Engine (if any at all?). How do I reference
that instance from the Node class.
I do not want doThis() to be a node method. I want to keep
doThis() as Engine method, as Engine manages many variant instances
of Node.
I would think something like this would be sufficient to pass
a node reference to doThis() from a node, as both Engine.as and
Node.as are in the same directory:
public function clickHandler(event:MouseEvent):void {
doThis(nodeID);
I have also tried: Engine.doThis(NodeID);
Again, how exactly do I reference an Engine function from
Node?