This set has me absolutely confused. Can anyone help with the first problem?

The other endpoint is ( -1,15)

Step-by-step explanation:

21.

 The x coordinate of the midpoint is the  sum of the x coordinate of the endpoints  divided by 2

(-5 +x)/2 = -3

Multiply each side by 2

-5+x = -6

Add 5 to each side

-5+x+5 = -6+5

x = -1

The y coordinate of the midpoint is the  sum of the y coordinate of the endpoints  divided by 2

(-1 +y)/2 = -8

Multiply each side by 2

-1+y = -16

Add 1 to each side

-1+y+1 = -16+1

y = -15

The other endpoint is ( -1,15)