A solid disc of radius ' $a$ ' and mass ' $m$ ' rolls down without slipping on an inclined plane making an angle $\theta$ with the horizontal. The acceleration of the disc will be $\frac{2}{b} g \sin \theta$ where $b$ is $\_\_\_\_$ . (Round off to the Nearest Integer) (g = acceleration due to gravity) ( $\theta=$ angle as shown in figure)