To convert the polar equation $r = 9 \cos \theta$ to rectangular coordinates, use the following transformations:

1. $x = r \cos \theta$
2. $y = r \sin \theta$
3. $r^2 = x^2 + y^2$

The equation $r = 9 \cos \theta$ can be rewritten using $\cos \theta = \frac{x}{r}$:

$r = 9 \cdot \frac{x}{r}$

Multiply both sides by $r$:

$r^2 = 9x$

Substitute $r^2 = x^2 + y^2$:

$x^2 + y^2 = 9x$

Rearrange to get the general form:

$x^2 + y^2 - 9x = 0$

This is the rectangular form of the given polar equation.