Iam sure there is a better way than this (I am newbie) using some build-in function, but how about this:

restart;
vars:={c1,c2,c3,c4,c5,c6,c7,c8,c9}:
e1 := c1 + 2*c2 + (c3+3*c4)*x*y + (c5+c6)*y^2 +(c8-2)*y^3*x^2 = 0:
e2 := (c4 + 2*c5 + c7)*x + (c9+ 2*c3+5*c4)*x^2 + (2*c7+5*c5+c6)*y^2*x^3 =0:
sys := map(coeffs,lhs~({e1,e2}),{x,y}) =~ 0:
print~(sys)[];

                         c1 + 2 c2 = 0
                         c3 + 3 c4 = 0
                          c5 + c6 = 0
                           c8 - 2 = 0
                       c4 + 2 c5 + c7 = 0
                      2 c7 + 5 c5 + c6 = 0
                      c9 + 2 c3 + 5 c4 = 0           

solve(sys,vars);

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));