The (-1/2,-1/2) is not a solution. It's easy to verify that the cannon can be at two locations by drawing circles (of arbitrary but large enough radius) around palm and coconut and checking the 2 intersections (*). But regardless of where the cannon is, the treasure will be to the "right" of the vector drawn from palm to coconut. Of course if you place the coconut tree at (0,-1), the treasure will be at (-1/2,-1/2) but that will still be the single solution and it will still be to the "right" of the aforementioned vector.
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