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): [tex] \frac{-24.3 m/s}{17.2 s} [/tex]=-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=[tex] \frac{-2170 kg m/s^2}{-1.41279 m/sec^2} [/tex]=1535.9678 kg