Using the given points, determine the slope. (5,-6)and(2,15)

Subtract the y-coordinates. This is the rise.
Subtract the x-coordinates in the same order. This is the run.
Slope = rise/run.

Subtract the y-coordinates: -6 - 15 = -21. rise = -21
Subtract the x-coordinates in the same order:5 - 2 = 3. run = 3

slope = rise/run = -21/3 = -7

It is volume.  The density would be far greater for the books and the atoms as well probably.  But volume is just the length x width x height or something.