Log2 (1-x) = 3 - log2 (3-x).

1-x >0; x < 1

3-x >0; x < 3

и тогда x <1

ОДЗ

log2 (1-x) = 3 - log2 (3-x).

log2 (1-x) = 3log2 (2) - log2 (3-x).

log2 (1-x) = log2 (2^3) - log2 (3-x).

log2 (1-x) = log2 (2^3 / (3-x).)

1-x=2^3 / (3-x).) = 8 / (3-x)

(1-x) (3-x) = 8

3 - x - 3x + x^2 - 8 = 0

x^2 - 4x - 5 = 0

D = (-4) ^2 - 4*1 * - 5 = 36

x1 = 1/2 (4 - 6) = - 1 входит в ОДЗ

x2 = 1/2 (4 + 6) = 5 НЕ входит в ОДЗ

ответ - 1