Here is the question: The only force acting on a 2.9 kg body as it moves along the positive x axis has an x component Fx = - 6x N, where x is in meters. The velocity of the body at x = 3.0 m is 8.0 m/s. a)What is the velocity of the body at x = 4.0 m? and b) At what positive value of x will the body have a velocity of 5.0 m/s? It does provide a hint : "Work is equal to the change in the kinetic energy. How does work by a spring force depend on the position? What is the value of the spring constant here?" So I am thinking that K= 6. And that work of the spring = (1/2)(k)(x_initial)^2 - (1/2)(k)(x_final)^2, and we can plug that in for work = (1/2)(m)(v_final)^2 - (1/2) (m)(v_inital)^2 to find the velocity when x = 4. so where did I go wrong here, how do i properly solve this?