Given that force applied due to tree on car is 6000 N
and during the stop the acceleration is given as 2 m/s^2.
So here we can say that As per Newton's II law

here F = force on car = 6000 N
a = acceleration of car = 2 m/s^2
now we can plug in all values above in the given equation


So here the mass of the car will be 3000 kg