Answer
250 km.
Work Step by Step
Call the distance d. The S-waves take a time $t_s=\frac{d}{v_S}$ to travel that distance. The P-waves take a time $t_P=\frac{d}{v_P}$ to travel that same distance.
We are told that the time delay is 33s. The faster P-waves arrive first, in less time.
$ \frac{d}{v_S}=\frac{d}{v_P}+33s$
$ \frac{d}{3.5km/s}=\frac{d}{6.5km/s}+33s$
$d = 250 km$
