Chapter P - P.3 - Functions and Their Graphs - Exercises - Page 29: 85

$$y=-\sqrt- x$$

First solve for y $$y^2=-x$$ Next take the square roof on both sides $$y=\sqrt -x$$ Since the bottom half of the parabola is the part that will have negative y values you will have to negate the root of -x $$y=-\sqrt -x$$