Answer:
K = m g (h₀ - h_plat)
Explanation:
Let's use energy conservation to solve this problem, write the energy at two points of interest
Initial. Higher
E_initial = m g h₀
Final. Lower
E_end = K + Ep
E_end = ½ m v² + m g h_plat
Energy is conserved
E_initial = E_end
m g h₀ = K + m g h_plat
K = m g (h₀ - h_plat)