Hey! So for problems related to energy exchange, we know that in STP (standard temperature and pressure), our standard pressure is 1 atm. So when we are given 931 mmHg, we need to convert them into atm in order to solve the problem. We know that 760 mmHg are equal to 1 atm. So our conversion should look something like this:
931 mmHg x 1 atm/ 760 mmHg = 1.225 atm
Remember that mmHg must be the denominator so it can cancel out with the mmHg that are at the beginning of the conversion.
Hope that helped!