What is the name of the part of the expressway where cars can both enter and exit?

Agas contained in a piston-oylinder assembly is initially at p1=100 ibf/ft^2? and occupies a volume of 3.75 ft. the gas is compressed to a pressure p2=500 ibf/ft^2 and a final volume of 0.5 ft. during the process, the relationship between pressure and volume is given as p= a +bv, where a and b are constants a)-determine the values and units for a and b b)-determine the volume, in ft, at an intermediate state where the pressure is 300 ibt/t2.