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
The substitution property of equality, one of the eight properties of equality, states that if x = y, then x can be substituted in for y in any equation, and y can be substituted for x in any equation.