12:(7-x)=6×(x-11)
12:(7-x) = 6x - 66
12 = 6x - 66 × (7-x)
12 - (6x + 66) × (7-x) = 0
12 - (42x-6x²-462+66x) = 0
12 - (108x -6x² - 462) = 0
12 - 108x + 6x² + 462 = 0
474 - 108x + 6x² = 0
6(79-18x+x²) = 0
79 - 18x + x² = 0
x² - 18x + 79 = 0
D = (-18)² - 4 × 1 × 79 = 8 > 0,
x1 = (18+2√2) / 2 = 9+√2
x2 = (18-2√2) / 2 = 9-√2