In the first place I congradulate you on desire to learn physics. It's well known to be the hardest thing on earth to master. Second, there is insufficient information to solve all but the mass of the car, which you did correctly. I recommend that the next time you first leave all the variables in until you do the final calculation. E.g. If M - mass of truck and m = mass of car, v_initial is the initial speed of truck and v_final is the final speed of the truck-car system then the mass of the car is m =(v_initial/v_final - 1)M = (4/2.5 - 1)6x10^{3}kg = 3.6x10^{3}kg.

You made an error in your calculation of kinetic energy loss. Please recheck your calculation. Since energy is a conserved quantity do you know where the kinetic energy went to?

The value for the impulse seems okay to me.