Draw a coordinate system and put the points into it. This is the easiest way to understand this. You can simply measure two sides of the triangel after this by looking at your drawing, then use the Pythagorean theorem to get the length of the last side.
Another method to get the lentgth of two sides:
By looking at the coordinates you can see that (-5,2) and (7,2) gives one side, which is 12 long (the length from -5 to 7) and (7,2), (7,-3) gives another side, which has a length of 5 (from 2 to -3). Use the Pythagorean theorem to get the last side:
5^2 + 12^2 = x^2
25 + 144 = x^2
169 = x^2
13 = x
You need the perimeter so simply add all the sides together: 12+5+13= 30
