[tex]A)\\\\x^3-7x^2+12x=0\\\\x(x^2-7x+12)=0\\\\x^2-7x+12=0\ \ lub\ \ x=0\\\\a=1,\ \ b=-7,\ \ c=12\\\\\Delta =b^2-4ac=(-7)^2-4*1*12=49-48=1\\\\\sqrt{1}=1\\\\x_{1}=\frac{-b-\sqrt{\Delta }}{2a}=\frac{-(-7)-1}{2*1}=\frac{7-1}{2}=\frac{6}{2}=3\\\\x_{2}=\frac{-b+\sqrt{\Delta }}{2a}=\frac{-(-7)+1}{2*1}=\frac{7+1}{2}=\frac{8}{2}=4\\\\x\in \left\{ 0,3,4\right\}[/tex]
[tex]B)\\\\ -2x^4+9x^3+5x^2=0\\\\x^2(-2x^2+9x +5 )=0\\\\-2x^2+9x+5=0\ \ lub\ \ x=0\\\\a=-2,\ \ b=9,\ \ c=5\\\\\Delta =b^2-4ac=9^2-4*(-2)*5=81+40=121\\\\\sqrt{\Delta }=\sqrt{121}=11\\\\x_{1}=\frac{-b-\sqrt{\Delta }}{2a}=\frac{-9-11}{2*(-2)}=\frac{-20}{-4}=5\\\\x_{2}=\frac{-b+\sqrt{\Delta }}{2a}=\frac{-9+11}{2*(-2)}=\frac{ 2 }{-4}= -\frac{1}{2}\\\\x\in\left\{-\frac{1}{2},0,5 \right\}[/tex]
[tex]C)\\\\x^3+3x^2+2x=0\\\\x(x^2+3x+2)=0\\\\x^2+3x+2=0\ \ lub\ \ x=0\\\\a=1,\ \ b=3,\ \ c=2\\\\\Delta =b^2-4ac=3^2-4*1*2\=9-8=1\\\\\sqrt{\Delta }=\sqrt{1 }=1 \\\\x_{1}=\frac{-b-\sqrt{\Delta }}{2a}=\frac{ -3-1}{2*1}=\frac{-4}{2}=-2\\\\x_{2}=\frac{-b+\sqrt{\Delta }}{2a}=\frac{-3+1}{2*1}=\frac{ -2 }{2}= -1\\\\x\in\left\{-2,-1,0 \right\}[/tex]