Let
be the height of the building and thus the initial height of the ball. The ball's altitude at time
is given by

where
is the acceleration due to gravity.
The ball reaches the ground when
after
. Solve for
:


so the building is about 16 m tall (keeping track of significant digits).
Set up the problem with the conversion rates as fractions where when you multiply the units cancel out leaving the desired units behind.
This is honestly not something I’ve learned. The answer is something I don’t know.