(assuming it starts from 0)
At start of 2nd second (t=1) it is at 13m, at end of 2nd second (t=2) it is at
30m, so it has moved 30-13=17m.
Now you should be able to do the 8th second part.
Average speed is 21m/s, distance travelled in 7 seconds is 7*21=147.An object moving with a velocity of 42m/s is stopped in 7 seconds.
How far has it travelled?
Isn't that 42+36+30+24+18+12+6 = 168? But answer is 147.
(using the aversge speed works here if we assume constant decelleration).