$$r_B = k[A]$$
$$K_‡ = \frac{[‡]}{[A]} \quad and \quad r_B = k_‡ [‡] $$
$$r_B = k_‡ K_‡ [A] \quad \Rightarrow \quad k = k_‡ K_‡$$
$$k = k_‡ K_‡$$