If an object is dropped from a tall building and hits the ground 3.0 s later, how tall is the building?

Let's take a couple of very common modelling assumptions, which you should definitely at this level take as standard:

Air resistance is negligible

Acceleration due to gravity is constant over small changes in height, relative to the size of the Earth

With these in mind, we just use Newton's Equations of Motion under constant acceleration. We want to solve for the displacement h, and the initial velocity is zero as 'dropped' implies from rest:

s = h
u = 0
v = ?
a = 9.81
t = 3.0

s = ut + 0.5at^2
= (0)(3.0) + (0.5)(9.81)(3.0^2) = 44m (2sf)

I hope this helps you :)

The charge of an ion with 55 protons and 52 electrons is +3. the reason is because protons are positive, and there are three more than the amount of negative electrons.