The impossible equation 2

$\dfrac{2a}{b+c}=\dfrac{2b}{a+c}$ $2a\left(a+c\right)=2b\left(b+c\right)$ $2a^2+2ac=2b^2+2bc$ $a^2+ac=b^2+bc$ $ac-bc=b^2-a^2$ $c\left(a-b\right)=\left(b+a\right)\left(b-a\right)$ $c=\dfrac{\left(b+a\right)\left(b-a\right)}{\left(a-b\right)}$ $=\dfrac{-\left(b+a\right)\left(b-a\right)}{-\left(a-b\right)}$ $=\dfrac{-\left(b+a\right)\left(b-a\right)}{\left(b-a\right)}$ $c=-\left(b+a\right)$ $\dfrac{2c}{a+b}=m$ $m=\dfrac{2\times -\left(b+a\right)}{a+b}$ $=2\times-1$ $=\boxed{-2}$