$3\mathrm{.}35$ The collar A slides on the smooth vertical bar. The masses

$m_A=20kg$ and $m_B=10kg.$ When $h=0\mathrm{.}1m,$ the spring is

unstretched. When the system is in equilibrium, $h=0\mathrm{.}3m.$

Determine the spring constant $k.$

Problem $3\mathrm{.}35$