A rock falls off a 15 meter cliff. How fast is the rock traveling vertically two seconds later?

The acceleration of gravity on Earth is 9.8 m/s². That means that

an object falling under the influence of gravity will move 9.8 m/s

faster than it was moving a second earlier.

Falling from rest, it will be moving 9.8 m/s after the first second,

and 19.6 m/s after the 2nd second.

The height from which it fell doesn't matter.