So, here is a reaction that we've talked about before, we talked about this proposed mechanism where we have a fast reversible step and a slow second step. So we learned last time that we can write the rate of product formation from the second step, there are two molecules of n o 2 being formed, so we have 2 times k 2, the concentration of n 2 o 2, and the concentration of o 2. But this is an intermediate, so we need to solve for that intermediate.