(x-7)(x+8)
(x-7)(x+8) = x²+8x-7x-56 = x²+x-56
x²+x-56 = 0
D = b² - 4ac = 1² - 4×1×(-56)=1+224=225
x₁ = 7
x₂ = -8
![\[{x_{12}}=\frac{{-b\pm\sqrt D}}{{2a}}\]](/tpl/images/0809/0950/92309.png)
![\[{x_{1}}=\frac{-1+\sqrt 225}{2*1}=\frac{-1+15}{2}=\frac{14}{2}=7\]](/tpl/images/0809/0950/26039.png)
