Answer
$(a)$ $y=kx$
$(b)$ $y=\frac{k}{x}$
$(c)$ $z=kxy$
Work Step by Step
*Note, $k$ is constant of proportionality
$(a)$ $y$ is directly proportional to $x$ $$y=kx$$
$(b)$ $y$ is inversely proportional to $x$ $$y=\frac{k}{x}$$
$(c)$ $z$ is jointly proportional to $x$ and $y$ $$z=kxy$$
