College Physics
posted by Jeff .
The only force acting on a 3.9 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?
(b) At what positive value of x will the body have a velocity of 5.0 m/s?
I solved for work by using the changes in kinetic energy and got 75 J but don't see how that helps whatsoever.

F = ma. They gave you Force and mass, solve for acceleration. You can use the values of X they give you, the velocities, and the acceleration to solve for Vf and the X value where v = 5.0

Yes I know of using that equation but the acceleration is not constant so I can't just plug in into an equation with a different x value.

truth.