You move a 2.5 kg book from a shelf that is 1.2 m above the ground to a shelf that is 2.6 m above the ground. what is the change in potential energy of a system containing the book and earth?

The change in potential energy of an object is given by

where
m is the mass of the object
g is the gravitational acceleration
is the increase in altitude of the object

In our problem,  is the mass of the book,  and 
is the increase in altitude of the book, so its variation of potential energy is