Displacement, velocity and acceleration are all vectors and therefore have a direction as well as a magnitude. Because we are considering only motion where the acceleration is constant (its direction is constant as well as its magnitude), it follows that all the motion takes place along a straight line. The object can, however, move either way along the line, so it is necessary to decide which is the positive direction; the opposite direction is then negative.

 

A good way to start is to make a list of the given information, and what is required, using the standard notation for initial and final velocities, displacement and time, giving each value its correct sign.