jmc

Since only the $y$ portions of the coordinates move, we know that the line of reflection must be a horizontal line. Now we just need to find the midpoint between an original point and its reflected image to pinpoint the location of the line. The $y$-coordinate of point $A$ is 3 and the $y$-coordinate of $A^{\prime }$ is $-5$; therefore, the midpoint is at $(2,-1)$. The line of refection is $\boxed{y=-1}$.