a)
[tex]x^3+5x^2-6x=0\\\\x(x^2+5x-6)=0\\\\x=0 \ \vee \ x^2+5x-6=0\\\\a=1, \ b=5, \ c=-6\\\\\Delta=b^2-4ac\rightarrow5^2-4\cdot1\cdot(-6)=25+24=49\\\\\sqrt{\Delta}=\sqrt{49}=7\\\\x=\frac{-5-7}{2}=-6\\\\x=\frac{-5+7}{2}=1\\\\x=\{-6,0,1\}[/tex]
b)
[tex]x^4-9x^2=0\\\\x^2(x^2-9)=0\\\\x^2=0\rightarrow x=0\\\\x^2-9=0\\\\(x+3)(x-3)=0\\\\x=-3 \ \vee \ x=3\\\\x=\{-3,0,3\}[/tex]