The change in x, from the given endpoint to the midpoint will be the same as the change in x from the midpoint to the missing endpoint.
The given endpoint has x = 1, the midpoint has x=-1 so we need to subtract 2 to go from 1 to -1.
So we do the same with the x value of the midpoint. subtract 2. -1 - 2 = -3. so the x coordinate of the unknown endpoint will be -3
the same idea applies to the y coordinates. Hopefully you can now work out the y coordinate of the unknown endpoint.