Proof when $b>1$, the zero point $(0,0)$ of ODE $\left\{\begin{align}&x'=x+y\\&y'=xy-bx-y\end{align}\right.$ is stable.
I couldn't find a proper Lyapunov V function.
Let $v=x'$ we has 1st order ODE $v\frac{dv}{dx}=xv-x^2-bx$, but it has no suitable expressions