Question: stationary points

I am trying to find the stationary ponts of f(x,y) 5*x^3-12*x-3*x*y+6*y^2-4;

so far, I have got

A:=diff(f,x)=0

B:=diff(f,y)=0

e1:=RootOf(A,x)

e2:=RootOg(B,x)

G:=solve([A,B],[x,y])

do I now need to use allvalues?

if so, I get

V:=allvalues(G). This gives me two values of x and y.  (one with a positive root and one with a negative root)

ie. v=[[x=c - sqrtb,y=4c - 4sqrtb],[x=c+sqrtb,y=4c+4sqrtb]]

r:=diff(f,x$2) =30x

s:=diff(f,x,y) = -3

t:=diff(y$2); = 12

now how do I sub in V[1] into the equation r*t - s^2? It wont work if I do

subs(V[1],r*t-s^2)

even if I sub these in manually, I only get two stationary values, but the question asks for values of f at its saddlepoints(plural) so I think I might've missed some solutions.. mybe with the sqrt?

many thanks in advance