Four isotropic point sources of sound that are uniformly spaced on an x-axis. The sources emit sound at the same wavelength and same amplitude

Four isotropic point sources of sound that are uniformly spaced on an x-axis. The sources emit sound at the same wavelength λ and same amplitude s_m, and they emit in phase. A point P is shown on the x-axis. Assume that as the sound waves travel to P, the decrease in their amplitude is negligible. What multiple of s_m, is the amplitude of the net wave at P if distance d in the figure is(a) λ/4, (b) λ/2 and (c) λ? 

S2 S3 S4 -dd