Can somebody help me with the derivation of an equation ?

Actually I am a PG student doing my thesis in Base Isolation systems. For the better understanding of what I need to do, I need to figure out the derivation of an equation from paper I refer. I tried all the reference paper from the author. But all the paper I referred is having the same equations that my base paper is having.

Check the link below for the paper I refered.
http://www.saifulamin.info/publicat...als/j8.pdf

I want the derivation of the Equation no.5 in page 5 of the above pdf.