Answer
Adding the attraction term and the actual pressure gives you the ideal gas pressure.
Subtracting a term from the actual container gets you ideal volume.
Work Step by Step
Ideal gases obey the KMT postulates, 3 of which are: gas particles take up no volume, particles collide with the walls of a container to create pressure, and there are no particle interactions.
In a real gas, molecules attract and interact with each other, which decreases the particle collision of walls, therefore making the pressure lower than the ideal. You have to add the factor of which the molecules interact with each other (the attraction term) to the actual pressure to get the ideal pressure.
In a real gas, molecules and particles have finite volumes, so the volumes available to a real gas is less than the container itself. Therefore, the apparent volume is lower than the actual volume, so you have to subtract a correction factor to get the ideal gas volume.
