Find the frequency of the sawtooth (voltage accross the capacitor)
Yes. I had to read your entire equation to see how Vss(t) was accounted for. To get the frequency of course you take the reciprocal of the time period, and your using the negative point of the waveform for a ref. [s] Is just a local reference for the waveshape. For these simple circuits, R and C1 dominate the total time period (and thus the frequency) while R2 and R3 adjust the trip point.