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Please do understand one thing here: Vds = -30V doesn't mean that any voltage in the circuit will be negative; it only indicates that the Drain voltage will have lower potential that the Source voltage.
The same with the Vgs: to open the Drain-Source channel the Gate voltage has to be lower than the Source voltage - see picture below.
So, if you connect the Source to, say, +12V and the Gate to any voltage below +10V (or so, data sheet will tell you) the P-channel MOSFET will be ON .. and the Vgs voltage will be Vg-Vs --> +10V - (+12V) --> -2V ..