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Notice that the segment from P1 to the given midpoint is a vertical line.
It is so, because first coordinate x= -3 is the same for both points P1 and the midpoint.
It means that the second endpoint has the same coordinate x= -3, and half of the problem is just solved in this way.
Thus you only need to determine y-coordinate of the second endpoint, which is easy now, after my hint.
Answer. P2 = (-3,-6).
