[질문] (7의n승-1)-(7의n-1승-1)=6·7의n-1승 의풀이과정

7^n은7*7^n-1로고칠수있죠?그렇게해서풀면뒤에마이너스일은빼기에의해사라지구요(7*7^n-1)-(7^n-1)이되니까같은거묶어내면(7-1)(7^n-1)=6*7^n-1이되겠네요^^