Nonlinear Systems & Linearization · Easy problem

For the system $x' = y,\; y' = x - x^3$, the energy $E(x, y) = \dfrac{y^2}{2} - \dfrac{x^2}{2} + \dfrac{x^4}{4}$ is conserved. Find the value of $E$ on the trajectory through $(2, 1)$.