Решим уравнения:
$$ (x+7)^3 = 216 $$
$$ x+7 = \sqrt[3]{216} $$
$$ x+7 = 6 $$
$$ x = 6 - 7 $$
$$ x = -1 $$
$$ (x+5)^7 = 1 $$
$$ x+5 = \sqrt[7]{1} $$
$$ x+5 = 1 $$
$$ x = 1 - 5 $$
$$ x = -4 $$
$$ (x+3)^9 = 512 $$
$$ (x+3)^9 = 2^9 $$
$$ x+3 = \sqrt[9]{512} $$
$$ x+3 = 2 $$
$$ x = 2-3 $$
$$ x = -1 $$
Найдем сумму корней уравнений:
$$ -1 + (-4) + (-1) = -1 - 4 - 1 = -6 $$
Ответ: -6