√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