We notice that we have a minus sign and $x^2$ is first. This suggests that we should use the secant function. We want $x^2$ to be $4\sec^2\theta$, so that $x^2 - 4$ becomes $4\sec^2\theta - 4 = 4(\sec^2\theta - 1) = 4\tan^2\theta$. This means that $x =
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