A stone is dropped at t $-$ O$.$ A second stone, with $3$ times the mass of the first, is dropped from the same point at t $=130$ ms$.($a$)$ How farbelow the release point is the center of mass of the two stones at t $=340$ ms$?($Neither stone has yet reached the ground.$)($b$)$ How fastis the center of mass of the two$-$stone system moving at that time?