The midpoint coordinates are: (8,-2)
We can label this coordinates as follows

The coordinates for one of the endpoints are: (3,10)
We label this coordinates as follows:

We are looking for the other endpoints with the coordinates (x2,y2).
We use the formulas to find the midpoint:

But, since we need x2 and y2, we solve for then in the equations:

And we substitute our values into the equations:
For x2:

For y2:

Answer: (13,-14)