What am I doing wrong? I've tried every conceivable answer, I thought I knew this type of math well, but this program keeps telling me I'm wrong.

Answer:
15.0 units
Step-by-step explanation:
Here, we want to get the distance between the two points as follows;
CD = √(x2-x1)^2 + (y2-y1)^2
So we have;
CD = √(7+8)^2 + (-5+4)^2
CD = √225+ 1 =
CD = √(226
CD = 15.0 units