Hooke's Law Direction of Force

[thomas49th]

Homework Statement

Hi I know for Hooke's Law F=-kx where -ve sign implies a restoring force back to equilibrium in the opposite direction of x.

My question is:
I have a mass being displaced attached to a spring - consider M2 and z2(t) and K2 in the link below http://gyazo.com/dfba23fbc16916940c25a02b1d96566e

So as z2 move downwards (in the image) the spring stretches and a the restoring force acts upwards. The total force is going to be the force from the spring plus the force from the displaced mass, which is

M_{2}\ddot{z_{2}} - K_{2}(\ddot{z_{2}} - \ddot{z_{1}}) = 0
note the -ve sign infront of the k2.

but why do we write a PLUS instead?

M_{2}\ddot{z_{2}} + K_{2}(\ddot{z_{2}} - \ddot{z_{1}})) = 0
(see answers below)

So, basically why do we not write

M_{2}\ddot{z_{2}} - K_{2}(\ddot{z_{2}} - \ddot{z_{1}})) = 0

as doesn't that make more sense. Because then the force of the spring restoration force equals the mass force, which is when the mass is at rest.The answers
http://gyazo.com/80ef4fc8e73dd510bd8c6489758471e7

 

[thomas49th]

come on someone surely must know, it's just a matter of signs