distance traveled by particle 'a' (s1)

s1=u1*t +1/2 *a1*t²

s1=3*t + t² / 2

distance traveled by particle 'b'(s2)

s2= u2 *t + 1/2*a2*t²

s2= t + t²

s1 - s2= 2t-t²/2

Minimizing s1 - s2 differentiate  the above equation and equate it with zero

we get t=2sec

put the value of t in s1 and s2 and get the required distance