Answer
The amplitude of the net wave at P is $~~4~s_m$
Work Step by Step
The path length difference to point P from any pair of sources is $~~n~\lambda~~$ where $n$ is an integer
Therefore, the four sources produce fully constructive interference.
The amplitude of the net wave at P is the sum of the amplitudes of each source.
Therefore, the amplitude of the net wave at P is $~~4~s_m$
