A 2.5 kg block at rest on a tabletop is attached to a horizontal spring having constant 19.6 N/m. The spring is initially unstretched. A constant 20 N horizontal force is applied to the object, causing the spring to stretch. Determine the speed of the block after it has moved 0.900 m from equilibrium if the surface between the block and tabletop is frictionless. I tried using the formula v= square root of k/m*(A^2-x^2) and got 2.52 which is the wrong answer. The 20 N force must need to be included in the problem in some way but I can't figure out how.