(1/3)^(3(x+21))=7^(x+21), (1/27)^(x+21)=7^(x+21), делим на 7^(x+21):
(1/(27*7))^(x+21)=1, x+21=0, x=-21
(1/3)^(3х+63) = 7^(х+21)
(1/3)^3·(х+21) = 7^(х+21)
(1/27)^(х+21) = 7^(х+21)
[1/(27 ·7)]^(х+21) = 1
[1/(27 ·7)]^(х+21) = [1/(27 ·7)]^0
Приравниваем показатели степеней при одинаковых основаниях
х + 21 = 0
х = -21.
