You measure that it takes 0.75 seconds for a leaf to fall from a tree to the ground. the leaf experiences air resistance as it falls. if there were no air resistance, how long would it take the leaf to fall?

There's not enough information given in the problem to calculate that answer.

A leaf falling from a tree on Earth, a sheet of printer paper falling off the back
of a truck on Venus, and a steel ball sinking through a bucket of Scotch whiskey
on Mars, might all reach the bottom in 0.75 second.  The time it would take each
of them to fall the same distance through a vacuum in the same place would be
different, and the 0.75 second is not enough to enable you derive it ... even if
you did know the acceleration of gravity in each place.  All you can say is that
without resistance, it would fall faster, and hit bottom in less than 0.75 second.