A voice signal is band-limited to 3.3 kHz. What is the Nyquist frequency?
If an analog signal is limited in frequency, and $W$ is its highest frequency component, then the minimum sampling frequency to avoid time-aliasing is $f_s = 2W$.

The Nyquist frequency is half the sampling rate of the system. 

In your case, and to avoid time-aliasing, the Nyquist frequency would be $f_{Nyquist} = 3 300\text{ Hz}$. But, since an additional guard band is considered, then the Nyquist frequency is:

$$f_{Nyquist} = 3 300\text{ Hz} + 1 400\text{ Hz} = 4 700\text{ Hz}$$