The answer is (4, -3) for The second endpoint.
In order to find this, we must first note that to find a midpoint we need to take the average of the endpoints. To do this we add them together and then divide by 2. So, using that, we can write a formula and solve for each part of the k coordinates. We'll start with just x values.
(Ax + Bx)/2 = Mx
(Bx + 10)/2 = 7
Bx + 10 = -14
Bx = 4
And now we do the same thing for y values
(Ay + By)/2 = My
(By + -1)/2 = -3
By + -1 = -6
By = -5
This gives us the final point of (4, -5)