A small object with mass m = 0.0900 kg moves along the +x-axis. The only force on the object is a conservative force that has the potential-energy function U(x) = −αx2²+βx³, where α = 8.50 J/m² and β = 0.300 J/m³. The object is released from rest at small x. When the object is at x = 4.00 m, what is the magnitude of its acceleration?