I know I have to show it does not satisfy the parallelogram law but I don't know how to apply it.
See the method described in the answer to your other question here. In particular, take $x = (1,0)$ and $y = (0,1)$.
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See the method described in the answer to your other question here. In particular, take $x = (1,0)$ and $y = (0,1)$.