Answer:
r = -4
Explanation:
If M is the midpoint of DE, that would mean that the distance from M to D and M to E would be the same.
This creates the equation 1-8r=13-5r.
Now it's just simple algebra.
You add 8r to both sides, creating 1=13+3r. Then, you subtract 13 from both sides, getting 3r=-12. Dividing both sides by 3 and solving for r, you get r=-4.
Checking our answer, you see that 1-8(-4)=1-(-32)=1+32=33, and 13-5(-4)=13-(-20)=13+20=33.