What does Destructive mean in terms of physics

Acube has a volume of 2011 cubic inches. what is the angle measure between a side of a cube and its interior diagonal?

Rohit is trying to tune an instrument

An electromagnetic wave with frequency 65.0 hz travels in aninsulating magnetic material that has dielectric constant 3.64 andrelative permeability 5.18 at this frequency. the electric fieldhas amplitude 7.20 \times 10^{ - 3} {\rm v}/{\rm m}. a. what is the speed of propagation of the wave? b.what is the wavelength of the wave? c.what is the amplitude of the magnetic field? d.what is the intensity of the wave?