Help with text book (vibration)

[MMCS]

HI,

I am studying vibration, came across this in a textbook and can't see the steps taken to substitute e.q 3.5 into 3.3. There is a second derivative x so where does this go, as there is no s^2 either and I can't see how cos was eliminated

Cheers

 

[soarce]

$cos^{\prime\prime} (y)=- \cos(y)$