Aforce of 2170 n is needed to bring a car
moving at 24.3 m/s to a halt in 17.2 s.
what is the mass of the car?
answer in units of kg.

It's implying that a constant force was applied throughout, which also means that there was a constant, negative acceleration. Because of that, the deceleration is equal to the change in velocity (-24.3 m/s) divided by the time (17.2 s): =-1.41279 m/s^2. The force is technically negative because it is causing a negative acceleration. Because F=ma: -2170kg m/sec^2=-(-1.41279 m/sec^2)m. m==1535.9678 kg