Под корнем х в квадрате+9 = 3 х-7

√x² + 9 = 3x - 7

x² + 9 = (3x - 7) ²

x² + 9 = (3x - 7) (3x - 7)

x² + 9 = 9x² - 42x + 49

x² + 9 - 9x² + 42x - 49 = 0

- 8x² + 42x - 40 = 0

8x² - 42x + 40 = 0

D = b² - 4ac = ( - 42) ² - 4 * 8 * 40 = 1764 - 1280 = 484 = 22²

x₁ = 42 + 22 / 16 = 64 / 16 = 4

x₂ = 42 - 22 / 16 = 20 / 16 = 1,25