[tex]81-2(x^2-2x+1)=51\;\rightarrow\;81-2x^2+4x-2-51=0\\
-2x^2+4x+28=0\;\;\;\Rghtarrow\;\;\Delta=16-4\cdot(-2)\cdot28=16+224=240\\
x_{1;2}=\frac{-4\pm\sqrt{240}}{-4}=\frac{-4\pm4\sqrt{15}}{-4}=\frac{-\not4(1\mp\sqrt{15})}{-\not4} =1\mp\sqrt{15}[/tex]
