Acar with a mass of 1,320 kg is at the top of a 50 m hill, 1. what is the velocity of the car? 2. how much gravitational potential energy does it have at the top of the hill? 3. how much kinetic energy does it have when it reaches the bottom of the hill?

1).  The car has no speed and no velocity.  Both of those are zero.
Until we get to part-3 of the question, the car is just sitting there at
the top of the hill.

2).  Gravitational potential energy is

                             (mass)  x  (gravity)  x  (height)

                     =  (1,320 kg) x (9.8 m/s²) x (50m)

                     =       (1,320 x 9.8 x 50)  kg-m²/sec²

                     =               646,800  joules.

3).  If there's NO friction on the way down, and NO air resistance,
then the kinetic energy it has at the bottom is exactly the potential
energy it had at the top ... 646,800 joules.      

NOW, THIS is where you might like to know the car's speed.

Its kinetic energy is 646,800 joules.  What is its speed ?

                           Kinetic energy  =  (1/2) (mass) (speed)²

                       646,800 kg-m²/s²  =  (1/2) (1,320 kg) (speed)²

Divide each side
by  660 kg :                (646,800 kg-m²/s²) / (660 kg)  = speed²

                                 Speed²  =    980  (m/s)²

                                 Speed  =  √(980 m²/s²)

                                            =    31.305 m/s  (rounded)

                                               (about  70 mph)

Amethod of procedure that has characterized natural science since the 17th century, consisting in systematic observation, measurement, and experiment, and the formulation, testing, and modification of hypotheses.