[tex]\it x_M=\dfrac{x_A+x_B}{2} \Rightarrow -4=\dfrac{a+(-2)}{2}|_{\cdot 2} \Rightarrow -8=a+2|_{-2} \Rightarrow \\ \\ \\ \Rightarrow a=-10 \Rightarrow A(-10,\ 3)\\ \\ \\ y_M=\dfrac{y_A+y_B}{2} \Rightarrow 1=\dfrac{3+b}{2}|_{\cdot 2} \Rightarrow 2=3+b|_{-3} \Rightarrow \\ \\ \\ \Rightarrow b=-1 \Rightarrow B(-2,\ -1)[/tex]
[tex]\it AB^2=(x_B-x_A)^2+(y_B-y_A)^2=(-2+10)^2+(-1-3)^2=64+16=80\\ \\ AB=\sqrt{80}=\sqrt{16\cdot5}=4\sqrt5\ dm\approx 4\cdot2,23=8,92\ dm[/tex]
