Answer
$$\tan(-\theta)=-\tan\theta$$
Work Step by Step
From the Negative-Angle Identities, we know that
$$\tan(-\theta)=-\tan\theta$$
Therefore, we should fill in the blank with $-\tan\theta$.
Actually, if you don't remember this identity, you can yourself make out by using the Quotient Identity: $$\tan(-\theta)=\frac{\sin(-\theta)}{\cos(-\theta)}=\frac{-\sin\theta}{\cos\theta}=-\tan\theta$$