Pls explain question no. 5. The answer is 6 m/s?

Dear student, 

Force= m × a
For initial case,
F=m × 2
F=2m.........eqn(1)

For final case,
Mass =3m
Acceleration =a2
Now,
F=3m × a2
using eqn 1, we have
2m= 3m × a2
So, a2= (2/3) m/s<sup>2

Regards</sup>