check the picture below.
thus
[tex]\bf \stackrel{0}{h}=-16t^2+64\implies 0=-16t^2+64\implies 16t^2=64\implies t^2=\cfrac{64}{16}
\\\\\\
t^2=4\implies t=\sqrt{4}\implies t=\stackrel{seconds}{2}[/tex]
as far as 5, is correct, however
[tex]\bf \textit{using the pythagorean theorem}\\\\
c^2=a^2+b^2\implies c=\sqrt{a^2+b^2}\qquad
\begin{cases}
c=hypotenuse\\
a=adjacent\\
b=opposite\\
\end{cases}
\\\\\\
c=\sqrt{15^2+8^2}\implies c=\sqrt{289}\implies c=17[/tex]
