How does destructive interference affect the amplitude of a wave?
The amplitude is reduced to zero.
Destructive interference means that two identical waves (same amplitude and wavelength) arrive in anti phase. In that case wherever wave 1 is at a peak wave 2 will be at a trough and vice versa. The displacement of a trough is equal in magnitude to that of a peak but it is negative, so the sum of the displacements will be zero.