Remember how a spring works: when you pull on it$,$ it streches. Newtons $3$rd Law of motion says that the spring pulls back on you too. If you pull the end of the spring in the $+$x direction with a force, F$,$ in the $+$x direction, the spring stretches y an amount x$.$ You know that the force has only an x component and hence Fx$+$kx where Fx is the force applied, k is the force constant of the spring and x is how much the spring stretches. What does Newtons #rd Law say about the force that the spring must exert on the hand stretching it$?($In one dimension you can use $+-$ to show direction