Write each equation in polar coordinates. Express as a function of $\small{t}$. Assume that $\small{r > 0}$.

(a) $\small{y = -6}$

$\quad \small{r =}$

(b) $\small{x^2 + y^2 = 7}$

$\quad \small{r =}$

(c) $\small{x^2 + y^2 - 7 x = 0}$

$\quad \small{r =}$

(d) $\small{x^2(x^2 + y^2) = 9 y^2}$

$\quad \small{r =}$