Now let's recall what the rate of reaction is. Well, the rate of any reaction or reactant is given by the following formula. Since we're going from reactants to products, our reactant concentration is decreasing, so the rate is negative. Change in x of concentration x divided by change in time is equal to our rate of forward. So k times our concentration of x. Now, since we're dealing with A equals zero, the zeros order, we plug in our zero for the exponent A.