Now notice that A-B-C and D are coefficients of each respective molecule and they represent the number of molecules or moles. Now, rates of reactions tell you how quickly reactants become products, so how quickly x and y become W and Z. In other words, the rate can be found by the change in concentration of reactants over some given time. Now, notice that we go from a positive amount or some amount to a small amount. So over time, our x and y will disappear. So therefore, the rate of these guys are negative because remember, we're subtracting our initial from final.