Two radio antennas $A$ and $B$ radiate in phase. Antenna $B$ is $120 \mathrm{~m}$ to the right of antenna $A .$ Consider point $Q$ along the extension of the line connecting the antennas, a horizontal distance of $40 \mathrm{~m}$ to the right of antenna $B .$ The frequency, and hence the wavelength, of the emitted waves can be varied. (a) What is the longest wavelength for which there will be destructive interference at point $Q ?$
(b) What is the longest wavelength for which there will be constructive interference at point $Q ?$