a rock 80m up the face of a cliff breaks loose.
About how long does it take for the rock to fall
to the ground?

Different books give different forms of the 
falling body formula. You didn't tell the way 
your book or teacher gives it so I'll just pick 
one. If it's not the form you are to use, post 
again giving the formula as your book or teacher 
gives it.

One form of the formula for the number 
of seconds it takes an object that is 
dropped from a height of h meters to 
fall to the ground is   
         _
t = 2.21Öh

So we just plug in 80 for h, and get a
calculator:
        __
t= 2.21Ö80

t = 19.8 seconds

Edwin