A1.5 kilogram cart initially moves at 2.0 meters per second. it is brought to rest by a constant net forcé in 3.0 second. what is the magnitude of the net force ?

The net force on the cart is

                   Force = (mass) x (acceleration) .

We know the mass = 1.5 kilogram, but we need to
calculate the acceleration.

               Acceleration = (change in speed) / (time for the change)

Change in speed = (ending speed) minus (beginning speed)

                               =    ( 0 )  -  ( 2 m/s )  =   -2 m/s

Acceleration =  (change in speed) / (time for the change)

                       =       ( -2 m/s )   /   (3 sec)

                       =           - 2/3  m/s² .

Finally ...        Net force = (mass) x (acceleration)

                                       =  (1.5 kilogram) x ( - 2/3 m/s²)

                                       =  ( - 3/2 x 2/3 )  (kg-m/s²)

                                       =       - 1  newton .

The negative sign on the force means the force is applied
opposite to the direction the cart is moving.  That gives
negative acceleration (slowing down), and the cart
eventually stops.   

Sorry i don’t know what to do