3.
[tex]M(\frac{x_A+x_B}{2},\frac{y_A+y_B}{2} )\\\\ M(-5,4)[/tex]
4.
[tex]AB=\sqrt{(1+4)^2+(-3-9)^2}=\sqrt{25+144}=13\\\\ BC=\sqrt{(-8-1)^2+(9+3)^2} =\sqrt{81+144}=15\\\\ AC=\sqrt{(-8+4)^2+(9-9)^2}=\sqrt{16}=4\\\\ P=AB+AC+BC=13+15+4=32cm[/tex]
