$(10$ points$)$ For the following differential equations, 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.

$($a$)\frac{d^{2}x}{d{t}^2}+6\frac{dx}{dt}+9x=4cos3t$

$($b$)2\frac{dx}{dt}=k(5-x)(1-x),$ where $k$ is a constant