\[v(t) = 4t - t^{2}.\]
\[1)\ 4t - t^{2} = 0\]
\[t^{2} = 4t\]
\[t = 4.\]
\[2)\ x(t) = 4 \bullet \frac{t^{2}}{2} - \frac{t^{3}}{3} = 2t^{2} - \frac{t^{3}}{3};\]
\[S = \left( 2 \bullet 4^{2} - \frac{4^{3}}{3} \right) - \left( 2 \bullet 0^{2} - \frac{0^{3}}{3} \right) =\]
\[= 32 - \frac{64}{3} = 32 - 21\frac{1}{3} = 10\frac{2}{3}.\]
\[Ответ:\ \ 10\frac{2}{3}\ м.\]