Answer:
Draw one dash mark between segment RS and another between ST.
Explanation:
Midpoint: The exact middle point of a segment that divides it into two equal halves.
To indicate if a point is the midpoint of a segment, make dash mark(s) to show on both sides of the midpoint it is dividing that they are congruent, or equal to each other.
Example in picture shown below:
Pretend that "A" is R, "M" is S, and "B" is T.
The green dash marks I made show that RS and ST are congruent.
There could be as many dash marks as you'd like, except both sides have to ALWAYS have the same number of dash marks to indicate they are congruent.
So, to show that point S is the midpoint of segment RT, draw one dash mark on both sides of the midpoint. Or draw as many as you'd like, but keeping the same amount of dash marks on both sides equal.