Go back and look at Math Essentials, page 99, and you will see the big table of Exponent rules --
1. z^4 over z (which is really z^1) is z^(4-1) which is z^3.
2. y^(-3) over y^3 is y^(-3-2) which is y^(-5). y^(-5) on top is the same as y^5 on bottom.
3. Generally, pieces stay where they are, unless we have a negative exponent, in which case, we switch from top to bottom. If we are reducing / canceling, the one with the bigger exponent is the one that stays and the one with the smaller exponent is the one that gets canceled.