You should try drawing a free body diagram. It reaaaally helped me in physics. If you did so you'll see that there are 2 forces acting on the object. The gravitational downwards pull and the resisting spring that's pulling it up. If you add those together by finding the difference you get a net force. Ask your teacher if you don't get any of these terms!
mg=4*9.8m/s^2=39.2N
kx=3*5=15N
Net force=39.2-15=24.2N
