Is it
(Ft2/2m)-2x
Two blocks mass m and 2m are kept on smooth horz surface and connected to each other using spring of "k".Initially spring is unstretched.A constant horz. force F applied to heavier block .Suppose at time t , displacement of smaller block is x, then find displacement of heavier block at this moment ?
F/3m should be acceleration of the c.m.
Displacemnt of the center of mass = 1/2 . F/3m . t2
let the disp of the larger mass be y
(2my+mx)/3m = 1/2 . F/3m . t2
(2my+mx) = 1/2 . F . t2
hence y = Ft2/4m-x/2