Answer
$\frac{-\pi}{2}\text{ radians}$
Work Step by Step
Recall that $2\pi\text{ radians}=360^\circ$, hence $\frac{\pi\text{ radians}}{180}=1^\circ$.
Therefore:
$-90^{^\circ}=-90 \cdot\frac{\pi\text{ radians}}{180}=\frac{-\pi}{2}\text{ radians}$
