Review the kind of line that has a zero slope.
A zero slope indicates that the difference in the y coordinates is always zero, regardless of the value of the x coordinate.
The formula for calculating the slope, m is
m = (y1 - y2)/(x1 - x2) = 0
The slope would be zero when the y coordinate is always the same value, in this case that value is negative three.
Look into it and then give it another try.