Linking two scale channels with a formula.

Hello Everyone,

I am trying to get myself familiar with Blender and have put together a simple animation of a jumping cube for my own education and amusement. I’d like to have the scaleX and scaleY channels on my cube object calculated based on the value of scaleZ. This way I can use the scaleZ channel to squash and stretch my cube while having the other scale channels automatically adjusted to maintain a consistent volume. Can anyone explain to me how to set this up (or point me in the right direction)?