y = 0
the equation of a line with zero slope is a line parallel to the x-axis and passing through all points with the same y-coordinate.
For this reason it's equation is y = c, where c is the value of the y-coordinate the line passes through.
the line passes through (- 3, 0 ) with y-coordinate 0, hence equation is
y = 0 ( that is the x-axis )