Momentum of a body is the product of its mass and velocity. In this case, the mass of the racecar is 750 kg and its initial and final velocities are 32 m/s and 15 m/s, respectively.
Hence, initial momentum = 750 kg x 32 m/s = 24,000 kg m/s
Final momentum = 750 kg x 15 m/s = 11,250 kg m/s
Change in momentum = Final momentum-initial momentum
= 11250-24000 kg m/s = -12,750 kg m/s
The negative sign indicates a reduction in momentum of the car.
Hope this helps.