eq:= (x+y)^2+ 1/(x + y) ;
algsubs((x+y)=m,eq);
simplify(%);
subs(m=(x+y),%);

f:=x->min(x^2 +1, 2*x+3) ;
plot({f(x),diff(f(x),x)},x=-3..3); %should also add labels, legend, etc....

If the determinant of the vectors is not zero, then the vectors are L.I. So simply take the determinant.

with(LinearAlgebra):
A := `<|>`(`<,>`(-1, -3, -6), `<,>`(3, 5, 6), `<,>`(-3, -3, -4));
v,e := Eigenvectors(A);
Determinant(e);
   

       -1

Hence L.I.

You do need to eventually specify an actual u(t) function

restart;

ode:=diff(x(t),t$2)+4*diff(x(t),t)+3*x(t)=u(t);

dsolve({ode,x(0)=0,D(x)(0)=0},x(t));