A man throws a ball of mass $m$ from a moving plank ($\mathrm{A}$). The ball has a horizontal component of velocity $u=3\sqrt{34} \mathrm{m} {\mathrm{s}}^{-1}$ with respect to ground towards another man standing on stationary plank ($\mathrm{B}$), as shown in the figure. The combined mass of plank $\mathrm{A}$ with man as well as that of plank $\mathrm{B}$ with the other man is $2m$ each. the x-y plane represents the horizontal plane which is frictionless. If initially, plank $\mathrm{A}$ was moving with speed $u\hat{\mathrm{i}}$ with respect to ground, then the relative velocity of plank $\mathrm{B}$ (in $\mathrm{m} {\mathrm{s}}^{-1}$) with respect to plank $\mathrm{A}$ (after the man on plank $\mathrm{B}$ catches the ball) minus $10 \mathrm{m} {\mathrm{s}}^{-1}$ is