Ok so these is no example in my book on how to do this and i missed this day of class. I understand how to get to a general equation of something like this but i don't know how to do the reverse. The problem is:

The equation of motion of a spring-mass system is given by

Determine the corresponding standard damped amplitude-phase lag expression for

.

I can't find that phrase "amplitude-phase lag" any where in my text so i have no idea what he's asking for. Can anyone give a little help? Thanks alot ^.^