Finding a strict Lyapunov function for a system of ODE's

Question

Regarding this question, of the following system : $$\begin{cases} \dot x=y-x^2-x \\\ \dot y=3x-x^2-y \\ \end{cases}$$ how can I prove that the equilibrium point $(1,2)$ is asymptotically stable using a Lyapunov function.
I chose my Lyapunov function to be $V(x,y) = a(x-1)^2 + b(y-2)^2 $, and with change of variables: $x=u+1 ,\\ y=v+2$
$\Rightarrow V(u,v) = au^2 + bv^2 $ and $\dot V=-2au^3-6au^2+2auv+2buv-2bu^2v-2bv^2$
But I can't determine what $a ,b$ should be in order to receive a strict Lyapunov function.
Would appreciate any help. Thanks:)