f(x)=-\frac{1}{2}x^2+6x-2
f(x)=8
-\frac{1}{2}x^2+6x-2=8
-\frac{1}{2}x^2+6x-10=0
a=-\frac{1}{2} , b=6 , c=10
\Delta=b^2-4ac=36-4*(-\frac{1}{2})*10=36-20=16
\sqrt\Delta=4
x_1=\frac{-b-\sqrt\Delta}{2a}=\frac{-6-4}{2*(-\frac{1}{2})}=\frac{-10}{-1}=10
x_2=\frac{-b+\sqrt\Delta}{2a}=\frac{-6+4}{-1}=\frac{-2}{-1}=2