if your referring to the one where I applied Newton's second law, I assumed the system ws horizontal and that the mass 1 was pulled a distance x1 to the right and mass 2 was pulled a distance x2 to the right. Then the spring between the two masses would be stretched and that spring will pull...
This is a the representation of the two masses. .
Using Newton's second law I got the following equations assuming x1>>x2:
m*x1''(t) = -k(x1-x2)
m*x2''(t) = 0.5kx1-kx2
I put it in matrix form
m| x1''| = | -k k| *|x1|
|x2''| =| 0.5k -k| |x2|
After some simplification assuming...