A certain spring found not to obey Hooke's law exerts a restoring force Fx(x) = —ax — /3x2 if it is stretched or compressed, where a = 60.0 Him and /3 = 18.0 HIM2. The mass of the spring is negligible. (a) Calculate the potential-energy func-tion U(x) for this spring. Let U = 0 when x = 0. (b) An object with mass 0.900 kg on a frictionless, horizontal surface is at-tached to this spring, pulled a distance 1.00 m to the right (the +x-direction) to stretch the spring, and released. What is the speed of the object when it is 0.50 m to the right of the x = 0 equilibrium position?