Numerical Methods: Euler to Runge–Kutta · Superchallenge problem

The two-step Adams–Bashforth method $y_{n+1} = y_n + h\left(\dfrac{3}{2}f_n - \dfrac{1}{2}f_{n-1}\right)$ is applied to $y' = -10y$. What is the largest step size $h$ for which both roots of its characteristic equation satisfy $|\xi| \le 1$? Express your answer as a common fraction.