Lets take the x and the y values individually.
So the endpoint x is -6 and the midpoint x is -3. The difference between the two is 3 values.
So if -6 is one end of x, then -3+3 will be the other end, which will be 0
The x value of the other endpoint is 0.
Now lets look at the y value. The endpoint is 4 and the midpoint is 2. The difference between the two is 2 values.
So if 4 is one end of y, then the other end of y will be 2-2= 0.
So the y value of the other endpoint is 0.
SO the coordinates of the other endpoint will be (0,0)