$a$) $2^2 . 3^{3x} - 27^x = 9^5$
$⇔ 4 . 3^{3x} - (3^3)^x = (3^2)^5$
$⇔ 4 . 3^{3x} - 3^{3x} = 3^{10}$
$⇔ 3^{3x} . (4 - 1) = 3^{10}$
$⇔ 3^{3x} . 3 = 3^{10}$
$⇔ 3^{3x+1} = 3^{10}$
$⇔ 3x+1=10$
$⇔ 3x = 9$
$⇔ x = 3$.
Vậy $x=3$
$b$) $2^x : 2 = 256$
$⇔ 2^x : 2 = 2^8$
$⇔ 2^x = 2^8 . 2$
$⇔ 2^x = 2^9$
$⇔ x = 9$
Vậy $x=9$.
$c$) $(x^2)^4 . 16 = 2^{20}$
$⇔ x^8 . 2^4 = 2^{20}$
$⇔ x^8 = 2^{20} : 2^4$
$⇔ x^8 = 2^{16}$
$⇔ x^8 = (2^2)^8$
$⇔ x = 2^2$
$⇔ x = 4$
Vậy $x=4$.