We can find the midpoint by finding the average between the two points.
We have r(1, 4) and s(-3, 7). 
The formula for midpoint is 
.
Let's plug in 1 and -3 in 
 and plug in 4 and 7 in 
.

Simplify the top numbers so now we have:

Simplify even further and now we have:
-1, 11/2
So the midpoint of the line segment is (-1, 11/2).