Cho

- Hướng dẫn giải

Đặt: $x=a+b+c$ thì 

$A={\left(2x-3c\right)}^2+{\left(2x-3b\right)}^2+{\left(2x-3a\right)}^2$

$\begin{array}{l}=\left(4x^2-12xc+9c^2\right)+\left(4x^2-12xb+9b^2\right)+\left(4x^2-12xa+9a^2\right)\\ =12x^2-12\left(a+b+c\right)+9\left(a^2+b^2+c^2\right)\\ =12x^2-12x^2+9m\\ =9m\end{array}$