Verified answer

Первый способ :

[tex]\displaystyle\bf\\15 < 3-4x < 21\\\\\\15-3 < 3-4x-3 < 21-3\\\\\\12 < -4x < 18 \ |:(-4)\\\\\\-4,5 < x < -3\\\\\\x\in(-4,5 \ ; \ -3)[/tex]

Второй способ :

[tex]\displaystyle\bf\\\left \{ {{3-4x > 15} \atop {3-4x < 21}} \right. \\\\\\\left \{ {{-4x > 15-3} \atop {-4x < 21-3}} \right. \\\\\\\left \{ {{-4x > 12} \atop {-4x < 18}} \right. \\\\\\\left \{ {{x < -3} \atop {x > -4,5}} \right.\\\\\\Otvet \ : \ x\in(-4,5 \ ; \ -3)[/tex]