The force of attraction between things that have mass is called

Agas initially at p1 = 1 bar and occupying a volume of 0.5 liter is compressed within a piston–cylinder assembly to a final pressure p2 = 4 bar. (a) if the relationship between pressure and volume during the compression is pv = constant, determine the volume, in liters, at a pressure of 3 bar. (b) repeat for a linear pressure–volume relationship between the same end states. reference

Can anybody me with this? ? (picture included.)

How many neutrons does element x if it’s atomic number is 28 and its mass number is 87