(a)
We know that the product of perpendiculars drawn from two foci $S_{1}$ and $S_{2}$ of an ellipse $\frac{x^{2}}{9} + \frac{y^{2}}{16} = 1$ on the tangent at any point $P$ on the ellipse is equal to the square of the semiminor axis.
$\therefore\left( S_{1}M_{1} \right) \bullet \left( S_{2}M_{2} \right) = 16$