The two springs in series has an effective spring constant of 2.644 N/m. Guess what? (chicken butt) If you take 5.289 N/m and divide by 2, you get 2.6445 N/m. This is what you would expect.

The spring constant, k, is different for different objects and materials. It is found by carrying out an experiment. For example, the unloaded length of a spring is measured.