What happened to the -4w in the denominator of this problem  w^2-9 / w^2-4w-21 if the answer is w-3/w-7

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(w^2-9)/( w^2-4w-21) .... Given ((w+3)(w-3))/((w+3)(w-7)) ....Factor/Canceling (w-3)/(w-7) .... Solution