Does it cost that much to multiply? Maybe a few seconds...
As you see, variables can be replaced, anytime, anywhere. So...we replace the x and y in the xy with the values defined. Maybe I'm being a little too "professional" for your tastes, but here is what I am trying to say:
xy = -8 * 9
What did we do, you may ask? We replaced the variables with the defined values!
Assuming you know how to multiply:
-8 * 9 = -72
And there we go!