A mass $m$ hung on a vertical ideal spring (of spring constant $k$) is initially held at its highest position above equilibrium such that the spring is compressed by $x_1$. When released, the mass falls through a distance $2h$ such that the lowest point it reaches is when the spring is stretched by $x_2$.

Based on this, what is $x_2 - x_1$? Neglect energy loss through friction or air resistance.