I prefer to use a midpoint formula that deals with both coordinates at once. For points A and B, their midpoint (M) is
... M = (A + B)/2
We can solve this for B by multiplying by 2 and subtracing A.
... 2M = A + B
... 2M - A = B
For your midpoint M(0, -10) and your endpoint A(-4, -5), the other endpoint will be
... B = 2(0, -10) - (-4, -5) = (2·0 -(-4), 2·(-10) -(-5))
... = (4, -15)
