A coin is dropped. What is its displacement after 0.30 s.

Assuming this coin is on earth and that it wasn’t dropped forcefully:

Use the formula d = 1/2at^2. Rewriting using a=g and solving for height h gets us h = 1/2 (9.8) t^2.

In this case that would get that the change in height h is 0.5 (9.8) (0.3^2) = 0.441 m.