First, we need to find the initial and final pressures using the given relation $P = aV + b$.
Initial pressure, $P_i = aV_i + b = -1200(0.42) + 600 = -504 + 600 = 96\,\text{kPa}$
Final pressure, $P_f = aV_f + b = -1200(0.12) + 600 = -144 + 600 = 456\,\text{kPa}$
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