a 1200 kg car starts from rest, accelerates to a speed of 40.25 m/s at a time of 11.5 seconds determine the acceleration of the car

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

Change in speed = (speed at the end) - (speed at the beginning)

For this car, change in speed = (40.25 - 0) m/s

Acceleration = (40.25 m/s) / (11.5 seconds)

Acceleration = 3.5 m/s²

The way you've written the question, we don't need to know and really don't care what the mass of the car is, or what color it is, or how many horses she's got under the hood. We don't even really need to know that we're talking about a car.

Acceleration = change in velocity / time

The mass here isn't needed.

a = 40.25 - 0 / 11.5

a = 3.5 m/s^2

You can also think of acceleration as the rate of change of velocity: hence meters per second per second.