$\text{Maximum Intensity } I_{max} = (\sqrt I_1 + \sqrt I_2)^2 = (\sqrt I + 2\sqrt I)^2 = 9I$

$\text{Minimum Intensity } I_{min} = (\sqrt I_1 - \sqrt I_2)^2 = (2\sqrt I - \sqrt I)^2 = I$