On route to senior prom, a couple was cut off by another car. The couple traveling 21 m/s slammed on the breaks and slowed to a speed of 8 m/s in a time of 1.4 seconds. What was the couple’s acceleration?
1) -9.3 m/s^2
2) 9.3 m/s^2
3) 18 m/s^2
4) -18 m/s^2
5) answer not shown