$ \langle 4,4,0\rangle$
Work Step by Step
$$(u-2v)\times (u+2v)= u\times u+2 u\times v-2v\times u-4v\times v\\ = -4u\times v=4\langle 1,1,0\rangle\\ = \langle 4,4,0\rangle.$$
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