1: Use Dv = at = 13 m/s (only 2 significant figures), + the initial v = 4 to get 17 m/s.
3: Use Dv = at to solve for the time: t=24.5/g (about 2.5 s). Then use Dx = at2/2 to get the height.
5,6: The average velocity is just the displacement / time. Since the acceleration is constant, the final velocity will be twice the average. The acceleration is then the final velocity / time (or use x=at2/2).
7: h=at2/2.
9: 100 km/hr = 27.8 m/s, 80 km/hr = 22.2 m/s. The cheetah can run 833 meters. In 30 seconds, the gazelle can run 667 meters, so she better start 166 meters ahead. To get the answer on the web page, one must do the arithmetic at high precision and then keep the 3 significant figures at the end. I would have only kept 2 significant figures...
11: The car travels at the average velocity of 14 m/s for 12 s, covering 168 m.