text: There must be destuctive interference.\n\nNow, wavelength = 0.34 m\n\nPath difference is sqrt(x^2+144) - 12\n\nThis must equal an integral or half number of wavelengths.\n\nTake the simplest case of phase diff. = pi\n\nSo, path difference = 0.34/2 = 0.17\n\nSo, sqrt(x^2+144) = 12.17\n\nor x = 2.027 m