Answer
The phase difference is $~~1.71$
Work Step by Step
We can find the distance from $S_1$ to $x_p$:
$d_1 = \sqrt{(9.00~\lambda)^2+(20.0~\lambda)^2}$
$d_1 = 21.93~\lambda$
We can find the distance from $S_2$ to $x_p$:
$d_2 = \sqrt{(3.00~\lambda)^2+(20.0~\lambda)^2}$
$d_2 = 20.22~\lambda$
We can find the path length difference:
$d_1-d_2 = 1.71~\lambda$
The phase difference is $~~1.71~~$ as a multiple of $\lambda$
