A block of mass 0.39 kg connected to a spring with spring constant 30 N/m is oscillating on a frictionless horizontal surface. Its speed as it passes through its equilibrium position is 0.5 m/s. What is the total energy of the system in J?

at the equilibrium position, there is no energy stored in the spring, it's all in the kinetic energy of the block

KE = 1/2 m v²