How to calculate sample stiffness

hi i need to calculate the torsional stiffness of a sample, the data is in the excel file.

To work it out it = 1/samplek= 1/totalk - 1/machinek

the machine k was given as 42800NM/radian

the total k is the slope of the line which comes to y=658x

what i dont get is what am i meant to do with y=658x,

cheers any help would be great
Attached Files
 AL_1_2.xls (65.5 KB, 8 views)
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 I've done a question very similar to that, and if I remember correctly then the gradient was either one of the stiffnesses or the shear modulus. But it was a year ago and I've not done anything like that since so I'm a little rusty on torsion. Edit: it's come back to me slightly. You have to find the shear modulus of the material, right? The slope is the total stiffness. Use the given equation to find out the sample stiffness and then you can easily calculate the shear modulus.