F.4 physics (acceleration)

The 10th second refers to the time interval from 9 s to 10 s.

Use equation of motion: s = ut + (1/2)at^2
with u = 0 m/s, t = 9 s, s = s1 (say), a =?
hence, s1 = (1/2)a(9^2)
i.e. s1 = 81a/2 = 40.5a

Use the same equation again,
with u = 0 m/s, t = 10 s, s = s2, say, a =?
hence, s2 = (1/2)a(10^2)
i.e. s2 = 50a

s2 - s1 = 50a - 40.5a
but it is given that (s2-s1) = 19 m
i.e. 19 = 50a - 40.5a
a = 19/(50-40.5) m/s^2 = 2 m/s^2

it should be easy for a F4 student.
use the formula s=ut+at^2/2
so 19=a10^2/2
a=0.38ms-2