A boy throws a stone vertically up with a speed $5\mathrm{.}4$ m$/$s$.$ Assuming that the stone's motion is affected by the gravitaional acceleration only $($acceleration g$=9\mathrm{.}8$ms$-2).$ What is the displacement in m between the stone's position at time $6\mathrm{.}8$ s and its initial position? $($hint: displacement has a sign $"+"$ or $"-",$ the canvas system can accept signed input. upward direction is $"+"$ and the downward direction is $"-",$ gravitation acceleration is negative in sign$)$