I am working on an assignment, and my first task is to write this second order differential equation as a system of two first order equations. I have done this previously for special cases where f and u are linear functions (typically f(u') = b*u' and s(u) = ku), but I have no idea of what to do with this generalized equation. Can somebody help me?

mu ̈+f(u ̇)+s(u)=F(t), t>0, u(0)=U0, u ̇(0)=V0