Acar traveling 26 m/s starts to decelerate steadily. it comes to a complete stop in 13 seconds. what is it's acceleration. /s^2

To find acceleration you subtract the final velocity and subtract it from the initial velocity and you divide the difference by the total time.

so you take the final velocity which is 0 m/s because the car comes to a complete stop.

then you subtract that from the initial which is 26 m/s

in other words:

0m/s - 26 m/s = -26 m/s

then you divide it by the total time which is 13 s

-26 m/s / 13 s

so you get

-2 m/s^2

The objects are acted upon by only one force. gravity. mass times the acceleration of gravity (-9.81meters/second squared).