Numerical Methods: Euler to Runge–Kutta · Easy problem

The backward (implicit) Euler method uses the slope at the new point: $y_1 = y_0 + h\, f(t_1, y_1)$. Apply one step with $h = \dfrac{1}{3}$ to $y' = -3y$, $y(0) = 1$. Express your answer as a common fraction.