Answer:
9 units.
Explanation:
In a square, all sides have the same length. So in this question, the distance between any of these two points is the length of a side.
Distance between two points.
Point A(x1, y1)
Point B(x2,y2)
The distance is given by:

In this question:
I am going to draw the square
The length of the side can be the distance between any of these points:
A and B, B and C, C and D or A and D.
I am going to use A and D.
A(-8,-8)
D(1,-8)

The length of one side is of 9 units.9 units.