Question

A hanging mass m = 1kg on a l = 1 m long rod is raised at an angle θ and let go.
It collides heads on and elastically with a 2-kg mass that is attached to a horizontal spring with k = 200 N/m. After collision the spring is compressed by ∆x = 30 cm. Derive an expression for θ as a function of m, k, l, g, and ∆x. Make sure that dimensions are correct. Find the angle.