What is the restorative torque applied to the disk in a pulley system?

[hoho0304]

Hi everybody, I was doing my physics homework but I found that I cannot figure out how to do it, here is the question:

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I don't quite understand how to calculate the torsional spring constant, so explanation on part a is enough:D Thanks for your help!

 

[rude man]

If you turn the disk an angle Θ, what restorative torque is applied to the disk by the strings?

(Both strings can be stretched a certain amount, in general, before you turn the disk).