Answer
By an odd number of half-wavelengths: $\frac{1}{2}$ wavelength, $\frac{3}{2}$ wavelength, $\frac{5}{2}$ wavelength, etc.
Work Step by Step
This is discussed on pages 549-550, and illustrated in Figure 29.12. If one wave is delayed by a half-wavelength, the two waves will arrive out of phase, and cancel each other.
Delaying that wave by a whole number of wavelengths on top of the half-wavelength delay doesn't change anything; the two waves will still be out of phase.