The height of an object dropped from the roof of an eight story building is modeled by: $\displaystyle {h}{\left({t}\right)}=−{16}{t}{2}+{64}$, $\displaystyle {0}\le{t}\le{6.8}$. Here, $\displaystyle {h}$ is the height of the object off the ground, in feet, $\displaystyle {t}$ seconds after the object is dropped. The average rate of change of $\displaystyle {h}$ over the interval $\displaystyle {\left[{0},{6.8}\right]}$ represents