The change in potential energy of an object is given by
[tex]U=mg \Delta h[/tex]
where
m is the mass of the object
g is the gravitational acceleration
[tex]\delta h[/tex] is the increase in altitude of the object
In our problem, [tex]m=2.5 kg[/tex] is the mass of the book, [tex]g=9.81 m/s^2[/tex] and
[tex]\Delta h=2.6 m -1.2 m=1.4 m[/tex] is the increase in altitude of the book, so its variation of potential energy is
[tex]U=mg\Delta h=(2.5 kg)(9.81 m/s^2)(1.4 m)=34.3 J[/tex]
