You can find the perimeter of a triangle if you know the length of its sides, so you'd just add the length of the three of them. You can use the three points to find the length of its sides by using the distance formula $$d = \sqrt{(x_1 - x_2)^2 + (y_1 - y_2)^2}.$$ For example, the distance of the side between the vertices $D$ and $E$ is $$d = \sqrt{(5-(-1))^2 + (0-6)^2}= \sqrt{36 + 36} = \sqrt{72} \approx 8.5 $$