If a gas is cooled from 323.0 k to 273.15 k and the volume is kept constant what final pressure would result if the original pressure was 750.0 mm hg

Alright, so, this seems to be dealing with gas laws. 
 

You have temperature, volume (Though volume wont matter in this equation becasue it would cancel itself out) and pressure. 



Your first fraction is what you start with, your second is what you end with 



Now in order to find the unknown pressure, you will multiple the 273.13 on both sides (On the first side to get x alone and then on the next becasue what is done to one side must be done to the other) 

x = 634.2 mm Hg