[tex]8 - x = {7x}^{2} \\ {7x}^{2} + x - 8 = 0 \\ D = {1}^{2} - 4 \times 7 \times 8 = 1 + 224 = 225 \\ x1x2 = \frac{ - 1± \sqrt{225} }{2 \times 7} = \frac{ - 1±15}{14} \\ x1x2 = - 1 \frac{1}{7} \: ; \: 1[/tex]