The transformation rule (x,y) ---> (x,-y) tells us that the x coordinate stays the same while the y coordinate will flip from positive to negative (or vice versa)
A point like (x,y) = (2,3) becomes (x,-y) = (2, -3)
Something like (x,y) = (7,4) will turn into (x,-y) = (7,-4)
Finally the point (x,y) = (6,8) reflects onto (x,-y) = (6,-8)
The three points that make up the final answer are: (2,-3) and (7,-4) and (6,-8)
The reflection that happened is a reflection over the horizontal x axis.