Решение:
$$ (23 - x) \cdot 4 - 6 = 30 $$
$$ (23 - x) \cdot 4 = 30 + 6 $$
$$ (23 - x) \cdot 4 = 36 $$
$$ 23 - x = 36 : 4 $$
$$ 23 - x = 9 $$
$$ x = 23 - 9 $$
$$ x = 14 $$
Ответ: 14