$a$) $x:2=x:3$
$⇔ \dfrac{x}{2} = \dfrac{x}{3}$
$⇔ 3x = 2x$
$⇔ 3x - 2x = 0$
$⇔ x = 0$
Vậy $x=0$
$b$) $(4.x + 5) : 3 - 121 : 11 = 4$
$⇔ (4x+5) : 3 - 11 = 4$
$⇔ (4x+5) : 3 = 15$
$⇔ 4x + 5 = 45$
$⇔ 4x = 40$
$⇔ x=10$
Vậy $x=10$
$c$) $890 : x = 35$ dư $15$
$⇔ 875 : x = 35$
$⇔ x = 25$
Vậy $x=25$
$d$) $648 - 34.x = 444$
$⇔ 34x = 204$
$⇔ x = 6$
Vậy $x=6$.