Answer
170.7 g/mol
Work Step by Step
We have d = PM/RT or M = dRT/P. First we need to place everything in the proper units. We have 743 torr so we convert it to atm:
743 torr * (1atm/760 torr) = 0.978 atm
Then we convert the temperature from 12C to Kelvin by adding 273 so we have 285K.
Then we have M = dRT/P = (7.135 g/L * 0.0821 * 285K) / 0.978 atm = 170.7 g/mol