Arock is thrown down with an initial speed of 10.0 m/s from a bridge in the water. it takee 2.75 s for the rock to hit the water. what is the speed of the rock as it hits the water? how high is the bridge above the water?

I would ordinarily pass on a question as complicated as this one is. 
But the lure of a bountiful 5 points at the end was too strong to ignore.

-- The acceleration of gravity on Earth is 9.8 m/s².  That means that
no matter how fast or slow an object is already falling, it'll be falling
9.8 m/s faster 1 second later.  In 2.75 seconds, its speed increases
by (9.8 x 2.75) = 27 m/s.  Added onto the shove you gave it when you
threw it down, it's moving at 37 m/s when it hits the water.

-- The rock's falling speed was 10 m/s when you threw it, and 37 m/s
when it hit the water.  It's average speed was (1/2) (10 + 37) = 23.5 m/s. 
It spent 2.75 sec falling with that average speed. So the distance it fell
was (23.5 m/s) x (2.75 sec) = 64.6 meters.