so hmmm those are the vertices, check the picture below
thus
[tex]\bf \textit{distance between 2 points}\\ \quad \\
\begin{array}{lllll}
&x_1&y_1&x_2&y_2\\
% (a,b)
A&({{ -3}}\quad ,&{{ 7}})\quad
% (c,d)
B&({{ -6}}\quad ,&{{ -1}})\\
B&({{ -6}}\quad ,&{{ -1}})\quad
% (c,d)
C&({{ 9}}\quad ,&{{ -1}})\\
C&({{ 9}}\quad ,&{{ -1}})\quad
% (c,d)
A&({{ -3}}\quad ,&{{ 7}})\\
\end{array}\
% distance value
d = \sqrt{({{ x_2}}-{{ x_1}})^2 + ({{ y_2}}-{{ y_1}})^2}\\\\
-----------------------------\\\\[/tex]
[tex]\bf AB=\sqrt{[-6-(-3)]^2+[-1-7]^2}\implies AB=\sqrt{9+64}
\\\\\\
\boxed{AB=\sqrt{73}}
\\\\\\
BC=\sqrt{[9-(-6)]^2+[-1-(-1)]^2}\implies BC=\sqrt{225}
\\\\\\
\boxed{BC=15}
\\\\\\
CA=\sqrt{[9-(-3)]^2+[-1-7]^2}\implies CA=\sqrt{144+64}
\\\\\\
\boxed{CA=4\sqrt{13}}[/tex]
thus, the perimeter of △ABC is AB + BC + CA
