Elastic potential energy springs

a certain spring is found not to obey hooke's law; it exerts a restoring force Fx(x) = -ax - bx^2 if it is stretched or compressed, where a = 60.0 N/m and b = 18.0 N/m^2. The m of the spring is negligible. a) Calculate the potential - energy function U(x) for this spring. Let U = 0 when x = 0. b) An object with m .900 kg on a frictionless, horizontal surface is attached to this spring, pulled a distance 1.00m to the right (the x-direction) to stretch the spring, and released. What is the speed of the object when it is .50m to the right of the x = 0 equilibrium position?