So when a reaction rate is directly proportional to the concentration of the reactant, it is set to be first order with respect to that reactant. So in this particular case, because the exponent is one, this is a first order reaction. And that means the rate law or the rate of that particular reaction depends directly, is directly proportional to the concentration. So by doubling the concentration of A, we double the rate by quadrupling it, we quadruple the rate, by tripling it, we triple the rate and so forth. So if we have a direct correlation between the reactant concentration and the rate of the reaction, then that means that reactant is first order with respect to that particular reactant. Now to calculate the overall order of that particular reaction we have to sum up all the exponents in that particular chemical rate law.