21.log4(x + 3)-log4(x-1) = = 2-log48

Решим уравнение log₄(x + 3) - log₄(x - 1) = 2 - log₄8

log₄((x + 3)/(x - 1)) = log₄(4²) - log₄8

log₄((x + 3)/(x - 1)) = log₄(16/8)

(x + 3)/(x - 1) = 2

x + 3 = 2(x - 1)

x + 3 = 2x - 2

x = 5

ОДЗ: x + 3 > 0, x - 1 > 0

x > -3, x > 1

x > 1

Ответ: 5