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

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

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