$7x^3+12x^2-4x=0$

\(\Rightarrow x\left(7x^2+12x-4\right)=0\)

\(\Rightarrow x\left(7x^2+14x-2x-4\right)=0\)

\(\Rightarrow x\left[7x\left(x+2\right)-2\left(x+2\right)\right]=0\)

\(\Rightarrow x\left(7x-2\right)\left(x+2\right)=0\)

\(\Rightarrow\left[{}\begin{matrix}x=0\\7x-2=0\\x+2=0\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=0\\7x=0+2=2\\x=0-2=-2\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=0\\x=\dfrac{2}{7}\\x=-2\end{matrix}\right.\)

Vậy \(x\in\left\{0;\dfrac{2}{7};-2\right\}\)

a. ZnCl₂ + 2NaOH→Zn(OH)₂ + NaCl

b. Fe₂O₃ + 3H₂SO₄→Fe₂(SO₄)₃+ 3H₂O

A=3+3^2+3^3+...+3^2009

=>3A=3^2+3^3+3^4+...+3^2010

=>3A-A=3^2010-3

=>2A=3^2010-3

=>2A+3=3^2010

=>n=2010