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.