The points form a rhombus, so the total perimeter is 4 times the length of one side. The difference in coordinates G and H is ...
... H - G = (2.5-1, 4-6) = (1.5, -2)
Then the length of GH can be found from the Pythagorean theorem to be
... GH = √(1.5² + (-2)²) = √(2.25+4) = √6.25
... GH = 2.5
The perimeter is 4 times that, so is 4×2.5 = 10 units.
