[tex]x^6+7x^5-8x^4=0\\x^4(x^2+7x-8)=0\\x^4=0~\vee~x^2+7x-8=0\\x=0~\vee~x=1~\vee~x=-8[/tex]
[tex]\Delta=49-4(-8)=81\\\sqrt{\Delta} =9\\x_1=\frac{-7-9}{2} =-8\\x_2=\frac{-7+9}{2}=1[/tex]
