P1 and P2 are the pressures, and V1 and V2 are the volumes. So you take the first pressure and volume you are given and place them into the equation P1V1 so the first part of the equation would be 101000*0.5 = P2V2. You then rearrange the equation to find what you want, in this instance you would do 50500/0.25 = P2... therefore P2 = 2020000Pa or 2.02*10^6Pa