If you said u is equal to w minus 300, then what is du? If you wanted the differential, so if you wanna take the derivative of u with respect to w, you would say du with respect to w is equal to what? It is equal to one. The derivative of this with respect to w is one. Or, if you multiply both sides times dw, you have du is equal to dw. And so, this left-hand side right over here, this integral could be rewritten as the integral of one over u du. We define this as u, and we just saw that du is the same thing as dw.