>>1459148It's a question about interference patterns. Since the towers broadcast the same wave at the same time, and the distance is given at wavelength terms, you can consider distance as phase. The further you go, the greater the phase gets, since it takes a bit longer to reach. Kind of like how people say we see stars in their past, because it took time for their light to reach us.
Try to describe the signal at each point using the sum of two sines (or cosines if you prefer) in the form of sin(ωt+φ_1)+sin(ωt+φ_2). Remember that two signals with a phase shift of 180 degrees cancel each other out, and two signals with the same phase add up nicely.
