In Problems 1 - 12, a differential equation is given along with the field or problem area in which it arises. Classify each as an ordinary differential equation (ODE) or a partial differential equation (PDE), give the order, and indicate the independent and dependent variables. If the equation is an ordinary differential equation, indicate whether the equation is linear

or nonlinear.

$$5 \frac{d^{2} x}{d t^{2}}+4 \frac{d x}{d t}+9 x=2 \cos 3 t$$

(mechanical vibrations, electrical circuits, seismology)