a)
[tex]6x^3+9x^2=3x^4\\3x^4-6x^3-9x^2=0\\3x^2(x^2-2x-3)=0\\x^2(x^2-2x-3)=0\\\\x=0 \\\\x^2-2x-3=0\\x^2+x-3x-3=0\\x(x+1)-3(x+1)=0\\(x-3)(x+1)=0\\x=3 \vee x=-1\\\\x\in\{-1,0,3\}[/tex]
b)
[tex]x^4=4x^3+5x^2\\x^4-4x^3-5x^2=0\\x^2(x^2-4x-5)=0\\\\x=0 \\\\x^2-4x-5=0\\x^2+x-5x-5=0\\x(x+1)-5(x+1)=0\\(x-5)(x+1)=0\\x=5 \vee x=-2\\\\x\in\{-2,0,5\}[/tex]
