Would you please solve it real quick, that will be helpful and appreciated. Thank you. I will give you a good feedback too.

10) Two small identical speakers are connected (in phase) to the same source. The speakers are 3m apart and at ear level. An observer stands at X, 4m in front of one speaker as shown. If the amplitudes are not changed, the sound he hears will be least intense if the wavelength is what? as viewed from above 3 m speakers .X 4 m