Let x be the displacement at time t of an object in motion.
Given, x = kt3, where k is a constant of proportionality.
Let u be the initial velocity of projection of body and v be the velocity of the same body while passing downwards through point of projection.
The displacement of body s = 0.
It means that the final speed is independent of mass of the body.
Hence, both the bodies will acquire the same speed while passing through point of projection.