how do we find change in entropy for two points on a P V diagram which are not an isotherm
Since the change in entropy of the system only depends on the initial and final states of the system, you can design an isothermal reversible process which goes from the intial point to the final point indicated by the diagram and easily calculate its ΔS. It will be the same even if the actual process you are studying is not isothermal or reversible.
The temperature at the initial state may not be the same as the temperature in the final state. So the path may not be totally isothermal. But you have the right idea.
The OP needs to "dream up" a reversible path between the initial and final states (any reversible path will suffice) for which the heat flows are easy to determine, and then calculate the integral of dq/T for that reversible path. That will give the change in entropy.
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