Answer
between $-x_m$ and 0.
Work Step by Step
We can see on figure 15-20b that the velocity at point B is positive, yet not at the maximum. Recall that for simple harmonic motion, the most extreme velocities occur when $x=0$ and zero velocity occurs at $-x_m$ and $+x_m$. Therefore we can conclude that because the velocity is positive and not at it's maximum, the particle must be between $-x_m$ and 0.
