Answer
As a multiple of $\lambda$, the phase difference is $~~5.80$
Work Step by Step
We can find the path length from $S_1$ to $P$:
$L_1 = \sqrt{(26.0~\lambda)^2+(6.00~\lambda)^2}$
$L_1 = 26.68~\lambda$
We can find the path length from $S_2$ to $P$:
$L_2 = \sqrt{(20.0~\lambda)^2+(6.00~\lambda)^2}$
$L_2 = 20.88~\lambda$
We can find the path length difference:
$\Delta L = (26.68~\lambda) - (20.88~\lambda) = 5.80~\lambda$
As a multiple of $\lambda$, the phase difference is $~~5.80$
