The book's vertical position in the air is
[tex]y=78.4\,\mathrm m-\dfrac12gt^2[/tex]
where [tex]g=9.80\,\frac{\mathrm m}{\mathrm s}[/tex]. It reaches the ground when [tex]y=0[/tex], at a time [tex]t[/tex] such that
[tex]0=78.4\,\mathrm m-\dfrac12gt^2\implies t=4.00\,\mathrm s[/tex]
So it takes the book 4 seconds to reach the bottom. The given initial velocity is irrelevant since it only has a horizontal component; vertically, the book is starting from rest.
