Hello

So mA =10kg,mB=20kg and Force=30N.

Time =2s and intial velocity =0m/s

Total mass of centre of mass =m1+m2

=10+20 =30kg

Total force on centre of mass =F=30N

Internal tension will not affect centre of mass

Acceleration =force/mass

=30N/30kg = 1 ms^-2

Using 2nd equation of motion

s= ut +1/2at^2

S=0 +1/2 ×1×2^2

S=2m.

Good Luck.