Let Q = (x,y) be the other endpoint. It is unknown for now.
Looking at the x coordinates of P and Q, we see that they are -16 and x respectively. Adding these values up gives -16+x. Dividing that result by 2 gives (-16+x)/2. This result is exactly equal to the midpoint x coordinate, which is the x coordinate of M (0).
So we have this equation (-16+x)/2 = 0. Let's solve for x
The y coordinates of P and Q are 0 and y respectively. Add them up and divided by 2, then set the result equal to -16 (y coordinate of midpoint M) getting this equation (0+y)/2 = -16
Well if you use graph paper you can graph the points you already have and then you can find the slope of the line easiest that way, the next point would be. (-1,8)