Answer
$\frac{[NO]^{2}[Br2]}{[BrNO]^{2}}$
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 NO and Br2. NO has a coefficient of two so it is raised to the 2 power. Br2 has a coefficient of one so it is raised to the first power. The reactant is BrNO that also has a coefficient of 2 raising it to the second power. Therefore the rate constant is $\frac{[NO]^{2}[Br2]}{[BrNO]^{2}}$
