Chapter 12 - Section 12.6 - Radian Measure - Exercise - Page 417: 3

$0.367\text{ radians}$

To convert degrees radians to degrees, multply $\dfrac{\pi \text{radians}}{180^o}$ to the given degree measurement to obtain: \begin{align*} 21^o \cdot \dfrac{\pi\text{ rad}}{180^o}&\approx 0.367 \text{ radians}\end{align*}