$\text{a) 71 - (33 + x) = 26}$
$\text{⇒ 33 + x = 71 - 26}$
$\text{⇒ 33 + x = 45}$
$\text{⇒ x = 45 - 33}$
$\text{⇒ x = 12}$
$\text{Vậy x = 12.}$
$\text{b) 45 - (x + 9) = 6}$
$\text{⇒ x + 9 = 45 - 6}$
$\text{⇒ x + 9 = 39}$
$\text{⇒ x = 39 - 9}$
$\text{⇒ x = 30}$
$\text{Vậy x = 30.}$
$\text{c) (x + 7) - 25 = 13}$
$\text{⇒ x + 7 = 13 + 25}$
$\text{⇒ x + 7 = 38}$
$\text{⇒ x = 38 - 7}$
$\text{⇒ x = 31}$
$\text{Vậy x = 31.}$
$\text{d) 198 - (x + 4) = 120}$
$\text{⇒ x + 4 = 198 - 120}$
$\text{⇒ x + 4 = 78}$
$\text{⇒ x = 78 - 4}$
$\text{⇒ x = 74}$
$\text{Vậy x = 74.}$
$\text{e) [(6x - 39) ÷ 7] × 4 = 12}$
$\text{⇒ (6x - 39) ÷ 7 = 12 ÷ 4}$
$\text{⇒ (6x - 39) ÷ 7 = 3}$
$\text{⇒ 6x - 39 = 3 × 7}$
$\text{⇒ 6x - 39 = 21}$
$\text{⇒ 6x = 21 + 39}$
$\text{⇒ 6x = 60}$
$\text{⇒ x = 60 ÷ 6}$
$\text{⇒ x = 10}$
$\text{Vậy x = 10.}$
