V(s) = volume of cube for some side length s units If s = 2, then we can replace s with 2 to go from v(s) to v(2) So v(2) represents the volume of a cube when it has a side length of 2 units Saying v(2) = 8 is the same as saying "if the side length is 2 units, then the volume of the cube is 8 cubic units"
You have 2/4 of a cup of water, you pour 1/2 a cup of water into your bowl how much water is left. none 2/4 and 1/2 are the same so when you subtract you get 0