Two identical uniform discs roll without slipping on two different...

$K . {E}_{d} = K . {E}_{T} + K . {E}_{Rot}$

$= \dfrac{1}{2}m{v}^{2} + \dfrac{1}{2}{I}_{cm}{w}^{2}$
$= \dfrac{1}{2}m{v}^{2} + \dfrac{1}{2}\left(\dfrac{m{r}^{2}}{2}\right){\left(\dfrac{v}{r}\right)}^{2}$
$= \dfrac{3}{4}m{v}^{2}$
${K}_{A} + {W}_{g} = {K}_{B} = \dfrac{3}{4}{mv}_{1}^{2} + mg \times 30 = {K}_{B}$
${K}_{C} + {W}_{g} = {K}_{D} = \dfrac{3}{4}m{v}_{2}^{2} + mg \times 27 = {K}_{D}$
${K}_{B} = {K}_{D}$ (Conversation of energy)
$⇒ \dfrac{3}{4}m{\left(3\right)}^{2} - \dfrac{3}{4}m{v}_{2}^{2} + 3mg = 0$
$⇒ {v}_{2}^{2} = 49$
$⇒ v = 7$