Question: How apply special transformation ?

How i can apply this transfromation which is special and i can't undrestand how from that solution he did thus changes  and get the solution of ode  ,  i reached untill eq(3.5) but after that i can't  figure out what he did   and how we get eq(3.7)-(3.9) (3.12),(3.13) and how we can   tranform solution  for the  eq(3.5)

fp.mw

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