$a) 7,5 × x + x × 3,5 = 39,93$
$⇒ x × (7,5 + 3,5) = 39,93$
$⇒ x × 11 = 39,93$
$⇒ x = 39,93 ÷ 11$
$⇒ x = 3,63$
$Vậy$ $x = 3,63$
$b) x : 4 + x =3,75$
$⇒ x × \dfrac{1}{4} +x = 3,75$
$⇒ x × (\dfrac{1}{4} + 1) = 3,75$
$⇒ x × \dfrac{5}{4} = 3,75$
$⇒ x = 3,75 ÷ \dfrac{5}{4}$
$⇒ x = 3$
$Vậy$ $x = 3$