The coordinates of point t are given. the midpoint of line st is (5,-8). find the coordinate of point s if t is (5,-15)

The easiest way to approach this problem would be to graph it. However, since I cannot do that, I will explain how to find the coordinate over writing. So, take note that the midpoint is at x=5 and the point T is at x=5. This shows that the line in which we are looking at is vertical. So we already know the x coordinate of S. (5,y)
Now, the midpoint is the point directly in the middle of 2 points. We need to find the distance from the midpoint to one of the points. 
From -8 ---> -15. This is 7 units down from the midpoint. The other point S would be 7 units up from the midpoint.
-8 > -1. The coordinates of point S is (5, -1)
I hope this helps :-)