-21.0 is classified as a negative whole number :)
(it's also a multiple of -3)
Because of the negative symbol in front of it, we know the distance from zero is not positive. Any positive number under 21 that may be added to it will bring it closer to zero, instead of further away! (like adding a positive number to a positive number would!)
And because there is no remainder after it, it is classified as a whole number.