Convert each polar equation to rectangular form.
(a) $r \cos \theta=3$
Rectangular form:
(b) $r=-7$
Rectangular form:

Step 1 ：Given the polar equations $r \cos \theta=3$ and $r=-7$, we are to convert these to rectangular form.

Step 2 ：The polar coordinates (r, θ) can be converted to rectangular coordinates (x, y) using the equations: x = r cos θ and y = r sin θ.

Step 3 ：For the first equation, we substitute r cos θ with x to get the rectangular form. Thus, the rectangular form of the equation $r \cos \theta=3$ is $x = 3$.

Step 4 ：For the second equation, r = -7, we substitute r with sqrt(x^2 + y^2) to get the rectangular form. Thus, the rectangular form of the equation $r=-7$ is $x^2 + y^2 = 49$.

Step 5 ：\(\boxed{x = 3}\) is the rectangular form of the equation $r \cos \theta=3$.

Step 6 ：\(\boxed{x^2 + y^2 = 49}\) is the rectangular form of the equation $r=-7$.