The only force acting on a 2.3 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.
At what positive value of x will the body have a velocity of 5.0 m/s?
I'm not sure how to approach this. So far I have written down KE=(1/2)mv^2,
but I haven't the slightest idea where to go from here...

Question

The only force acting on a 2.3 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. 
At what positive value of x will the body have a velocity of 5.0 m/s? 
I'm not sure how to approach this. So far I have written down KE=(1/2)mv^2, 
but I haven't the slightest idea where to go from here...