Hello!
The slope of the line given by graphing pressure vs 1/Volume at constant temp for one mole of gas will give you the value for nRT from equation PV=nRT
So set nRT=slope and take the constant number mole of gas and the constant temp and solve for R the universal gas constant. You arm for pressure and litters for volume to get R in units of L*atm/mol*k
Hope this helps you! Thanks!!