A force of 20N is applied on an object. It moves whit a constant velocity, what is the force of the opposing force F?

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)