I've attempted to draw a free body diagram of the forces acting on the mass again, but I'm confused about the directions. In many of the sources that I've read, they say that the Fres = Fs + F. Fs is the spring force and F is the force from the oscillation generator. But considering when the mass is displaced maximum to the right, the spring of the left will (almost) have no effect since its tension reduce. Only the compressed spring on the right will exert a spring force on the mass, right? Also, when the mass is oscillating in phase with the oscillation generator, that is if the force pushes to the right, the mass will displace to the right. Therefore, why is Fres = Fs + F, and not Fres = F - Fs (if to the right is the positive direction)?

Moreover, the equation I get should be:

So how do I draw the free-body diagram to get that?