Having selected our operating point at 85V, we know anode, and hence, cathode current. (In triode anode and cathode currents are equal). We know the value of Vgk we need - it has to be 2V below the potential of the cathode. If the greed is at 0V, the cathode has to be at +2V. If we know the voltage across, and the current through, an unknown resistor, we can find the value of the resistor through Ohm's law.
We know that the cathode voltage is 2.0V, so the cathode bias resistor R3 will have value of 363 Ohm. The closest standard value is 360 Ohm.
