If the slope if is undefined, then the line is vertical.
The equation of a vertical line: x = a (a - any real number).
The line passes through the point (-3, 0) → x = -3 and y = 0.
Therefore we have x = -3.
The general form of a line: Ax + By + C = 0.
x = -3 add 3 to both sides
x + 3 = 0