a particle starts from rest with uniform acceleration a.its velocity after n seconds is v.the displacement of the body in last two seconds is:

the distance travelled by the particle in the last two seconds will be

 = distance travelled by particle in nth second - distance travelled by particle in (n-2)th second

so,

s = (1/2)an² - (1/2)a.(n-2)²

= (1/2)a.[n² - n² +4n - 4]

 = (a/2).[4n - 4]

so,

s = (2n-2).a = 2a.(n-1) ............................(1)

now,

velocity, v = acceleration x time

v = a x n 

or

a = v/n ..........................(2)

so, the displacement in the last two seconds will be

s = 2(v/n).(n-1)