The answer is (9, -9) for The second endpoint.
Let's start by calling the known endpoint L and the unknown K. We'll call the midpoint M. 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.
(Kx + Lx)/2 = Mx
(Kx + 5)/2 = 7
Kx + 5 = 14
Kx = 9
And now we do the same thing for y values
(Ky + Ly)/2 = My
(Ky + -3)/2 = -6
Ky + 3 = -12
Ky = -9
This gives us the final point of (9, -9)
