Let the amplitude of light wave coming from the narrower slit be $a \rightarrow $ the amplitude of light wave from the wider slit $ = 2a$

The maximum intensity occurs where the constructive interference takes place and the minimum intensity where the destructive interference takes place.

$\Rightarrow a_{max} = 2a + a = 3a$

$\Rightarrow a_{min} = 2a - a = a$

$\Rightarrow$ Ratio of maximum to minimum intensity $=\large\frac{I_{max}}{I_{min}}$$=\large\frac{a_{max}^2}{a_{min}^2}$$=\large\frac{3^2a^2}{a^2}$$=9$