Since you have 3 vertices, the polygon you have is a triangle. The sides of the triangle will be XY, XZ, and YZ. Just use the pythagorean theorem to calculate the length of each side and then add the lengths together for the result.
Side XY = sqrt( (-1-3)^2 + (3-0)^2) = sqrt(16+9) = sqrt(25) = 5
Side XZ = sqrt ( (-1-(-1))^2 + (3-(-2))^2) = sqrt(0+25) = sqrt(25) = 5
Side YZ = sqrt( (3-(-1))^2 + (0-(-2))^2) = sqrt(16 + 4) = sqrt(20) = 4.47
Now add the length of each side together
5 + 5 + 4.47 = 14.47</span>
