Aroller coaster car rapidly picksup seed as it rolls down a slope. as it starts down its speed is 4m/s. 3 seconds later its speed is 22 m/s. what is its average acceleration

Change in speed  = (end speed) - (start speed)

                             =    (22 m/s)    -   (4 m/s)   =   18 m/s .

Magnitude of average acceleration =

                         (change in speed) / (time for the change) .

                     =                (18 m/s)  /  (3 sec)  =  6 m/s² .

That's about 0.61 G.
If you weigh 150 pounds, then you would feel as if there's
a force of 92 pounds pushing you back against the seat !