Answer
The amplitude of the net wave at P is zero.
Work Step by Step
We can find the path length difference to point P from $s_1$ and $s_2$:
$\Delta L = \frac{\lambda}{2} = 0.5~\lambda$
Therefore, $s_1$ and $s_2$ produce fully destructive interference.
Similarly, $s_3$ and $s_4$ produce fully destructive interference.
Therefore, the amplitude of the net wave at P is zero.
