homework and exercises - Moments of vibrating string

I have to calculate first 10 moments for vibrating string with damping, but I don't know how to do it. I read about moments and in definition they mention linear system

$$x'=Ax+Bu, \qquad y=Cx$$ with transfer function $$W(s)=C(sI-A)^{-1}B.$$ Then kth moment of a system above is $$C(sI-A)^{-(k+1)}B.$$ But for a vibrating string I know only wave equation $$Mx''+Cx'+Kx=b(t).$$ How to transform it to linear system?