Kinematics equations are vital for this:
v_0(initial velocity) v_f(final velocity) a(acceleration)
v_0=70 km/h v_f= 0 km/h( rest)
Kinematics equation is a=(v_f-v-0)/time
a=(0-70)/(7/3600) which means it decelerates at -36000 km/h
This number is humongous; I would advise you to re-check the problem and see if the the time it takes is 7 hours rather than 7 seconds. Notice that I put the time as 7/3600 because it is 7 seconds which should be converted to hours.
