I have a class with a subclass. The superclass has a Position property. The subclass must perform an additional operation when the Position property is changed, so I am attempting to override the setter method and call the superclass' setter.
I think I've got the superclass setter calling part down, but I can't figure out how the overriding syntax works here.
Here is my best attempt:

The getter is there just for proof of concept -- suppose I wanted to override that too?
The getter and setter give me errors of this form:
cannot override inherited member 'superClassName.Position.[gs]et' because it is not marked virtual, abstract, or override
Here's a screencap of the errors too for good measure:

I also tried using the override keyword in front of the set. Removing the superfluous getter has no effect.
What is the correct syntax?