Question

A man is pushing a cart down a slope suddenly the cart starts moving fast and he wants to slow it down what should he do

Answer 1

To drown the gravitational force of the cart that leads it fast down the slope , the man should apply muscular force opposite to the force of the cart to slow it down.

Answer 2

When a man pushes the cart down the slope,he exerts some force,moreover since it is down the slop the gravitational force act as well,making it easier to move. [Hence it will move very fast]

To stop the cart,one has to apply a muscular force in opposite  direction,to the displacement f the cart,i.e basically u have to pull the cart.

This will reduce the effect of gravitational force acting,and the velocity will reduce

The other force acting to reduce the speed will be friction,friction acts opposite to the displacement of the object.