You didn't list a full equation. Is this is as far as you can simplify this with the info given. to get it to that point just subtract 2y from y. If you meant Y=z-2y then the answer would be 3y=z or Y=z/3 (the same things just more is more simplified). For a more complete answer gove more information.
In 52-card deck there are 36 numbered card and 52/4=13 spade card so probability that you are dealt a numbered card is 36/52 =9/13 a spade card is 13/52=1/4
