3.13 How to find the order of an ODE?

Use

Clear["Global`*"]; 
ode=y''[x]+3*y'[x]+3==0; 
Internal`ProcessEquations`DifferentialOrder[ode,{x},{y}] 
 
(*   {{2}} *)
 

So it is order 2. Notice the input must be lists [ode,{x},{y}], it will hang if you use [ode,x,y]