use hydrogen spectral line formula

use ? = ?D/d

Given the distance between the two towers = 40 km We know that for distances smaller than Fresnels distance(ZF) , spreading due to diffraction is smaller as compared to the size of the waves. But the hill is half way between the tower so distance upto which much of diffraction will not be observable should be 40/2 = 20km = 20,000m We know that Fresnels disatnce is the minimum distance a beam of light can travel before its deviation from straight line path due to diffraction becomes significant. Hence ZF = 20,000 m For the hill not to obstruct the spreading radio beam, the radial spread of the beam over the hill 20 km away must not exceed 50 m. / lambda= 50*50/ 20, 000 = 0.125 m. Therefore the longest wavelength of radiowaves which can be sent between the towers without appreciable diffraction = 0.125 m

use x=n ?1D/d