Answer
$-0.70\text{ radians}$
Work Step by Step
Recall that $2\pi\text{rad}=360^\circ$.
Hence, $1^\circ=\frac{\pi\text{ radians}}{180^\circ}$.
Therefore:
$-40^{\circ}=-40 \cdot 1^{\circ}=-40\cdot\frac{\pi\text{ radians}}{180^\circ}\approx-0.70 \text{ radians}$