So as the reaction is progressing from the reactant side to the product side, a is being consumed. And that's why the rate of A is negative and the rate of A is simply given by the change in the concentration of A with respect to some time interval and likewise B is also being consumed. So the rate of B is negative and it's given by delta the change in concentration of B with respect to the time and likewise over the same time interval. These are the rates of C and D. So the rate of C is the change in C with respect to T and the rate of D is change in D with respect to T. Now, these equations aren't very useful. What we actually want to be able to express is the entire rate of that entire equation as a whole. And this is where the rate law and the rate constant comes into play.