What is the formula for the potential energy of a simple harmonic oscillator in terms of the equilibrium position and the angle theta?

The formula for the potential energy of a simple harmonic oscillator in terms of the equilibrium position and the angle theta is U 1/2 k (x2 (L - x)2), where U is the potential energy, k is the spring constant, x is the displacement from the equilibrium position, and L is the length of the spring at equilibrium.