Verified Answer :

mathmate

Hint:
r_{y=k): (x,y)->(x,-(y-k)+k)
or simplified,
r_{y=k): (x,y)->(x,2k-y)

So in this case, k=-1, x=2, y=-4
(2,-4) -> (2, 2(-1)-(-4)
(2,-4) -> (2, -2+4)
(2,-4) -> (2, 2)

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