2x²+13x -56 ≤ (x-4)²
2x²+13x -56 ≤ x² - 8x + 16
2x² - x² + 13x + 8x - 56 - 16 ≤ 0
x² + 21x - 72 ≤0
Найдем нули функции y = x² + 21x - 72
x² + 21x - 72 = 0
По т.Виета
x1 + x2 = -21 -24 + 3 = -21
x1 * x2 = -72 -24 * 3 = -72
x1 = -24
x2 = 3
+ - +
----------- -24 ------------------- 3 ----------
x ∈[-24; 3]