The stone reaches a height of 13 m when [tex]h=13[/tex]:
[tex]8t-0.83t^2=13\implies t\approx2.069\text{ or }t=7.569[/tex]
The question refers to the moment at which it **rises** to 13 m, so we need to take the earlier instance, [tex]t\approx2.069[/tex] seconds after the stone is tossed.
The stone's velocity is given by the derivative of its height function:
[tex]v(t)=h'(t)=8-1.66t[/tex]
Plug in the time found above; the stone's velocity at this moment is about 4.57 m/s.
