I'm afraid you're wrong. It asks for the rate law [sc. of the reaction, what other kind is there?] consistent with the proposed mechanism, not the "rate law of the mechanism", whatever that may mean. Your equation is right for the slow step taken alone, but it is not legitimate as a rate law because it involves an intermediate, and you need the rate law in terms of the starting materials. As the first step is a fast equilibrium between NO, Br2 and NOBr2, you can express [NOBr2] in terms of [NO] and [Br2], and this is what you should have done.