Proportionality A statement describing the relationship between the variables $x, y,$ and $z$ is given. (a) Express the statement as an equation of proportionality. (b) If $x$ is tripled and $y$ is doubled, by what factor does $z$ change? (See Example $3 . )$
$$\begin{array}{l}{z \text { is directly proportional to the square of } x \text { and inversely pro- }} \\ {\text { portional to the fourth power of } y .}\end{array}$$