If the volume is 12 mL at 40 lb/in_{2}, then 1/V = (1/12) mL^{-1}.

If 12 mL refers to the initial volume, V_{1}, of the gas at an initial pressure P_{1}, and then the pressure has been changed to P_{2} (= 40 lb/in_{2}), you would have to calculate the corresponding new volume V_{2}.

The question you quoted implies there is a data set that you are not showing.

I can not help you any further unless you post the entire question exactly as you have it - as I have already asked.