Question

A pencil is dropped from the roof of the school what is the speed after 3s down is the negative direction.

I'm confused on what equation to use

Answer 1

you have to use 1st equation of motion v = u +at as distance(s) is not given.....
as pencil is dropped from rest therefore u = 0m/s and accleration due to gravity is = -9.8 m/s
v = 0 - (9.8)(3)
v = -29.4 m/s

Answer 2

On Earth, the acceleration due to gravity is (-9.8 meters per second) per second.

If the school, the roof, and the pencil are on Earth,
then after 3 seconds, the pencil's speed is

[ (-9.8 meters per second)/second ] times [ 3seconds ] = -29.4 meters per second