The force exerted by a spring is different from what we generally saw in Chapter 4, where we dealt with constant acceleration. Springs are more complicated. Not only does the magnitude of the spring force vary with distance (and therefore time), the direction of the force depends on whether the spring is being stretched or compressed.
The spring in the simulation below has a spring constant that you should be able to determine by dragging the mass.
Modified from Andrew Duffy's web page at Boston University. The animation uses the Physlet Java Applets of Wolfgang Christian and Mario Belloni.