Answer
$\frac{[SbCl3][Cl2]}{[SbCl3]}$
Work Step by Step
The equilibrium constant, k, is defined as the products to the power of the coefficient over the reaction to the power of the coefficient. In this case, the products are SbCl3 and Cl2, all with a coefficient of one. The reactant is SbCl3 that also has a coefficient of 1 giving the rate constant of $\frac{[SbCl3][Cl2]}{[SbCl3]}$
