What is the acceleration experienced by a car that takes 10s to reach 27m/s from rest

|acceleration| = (change in speed) / (time for the change)

Change in the car's speed = (27 - 0) = 27 m/s

Time for the change = 10 sec

|acceleration| = (27 m/s) / (10 s) = 2.7 m/s².

That's the magnitude of the car's acceleration.

We don't know anything about its direction.