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    Everyone uses midpoint formula, but we can use a different strategy instead:

    Notice that to rotate (x, y) around the origin we can use (-x,-y). Now all we have to do is move to the origin, perform reflection, and move back.

    Lets employ an algorithm using the coordinates (px, py) and (qx, qy): 1. We move (px,py) and (qx,qy) such that (qx,qy) is at the origin and their distance is maintained. This gives the new coordinates of (px,py) to be (px-qx,py-qy) 2. Now use the rule and multiply by -1, to get (-1 * (px-qx), -1*(py-qy)) 3. Now we go back to the original coordinate by adding back, to get (px + -1 (px-qx), py + -1(py-qy))