A hospital helipad is at H(2, 2). Four ambulance posts stand at A(5, 6), B(-2, 5), C(6, -1) and D(7, 2), all distances in kilometres.
What is special about the four posts?

$HA=\sqrt{(5-2{)}^2+(6-2{)}^2}=\sqrt{9+16}=5,HB=\sqrt{(-2-2{)}^2+(5-2{)}^2}=\sqrt{16+9}=5,HC=\sqrt{(6-2{)}^2+(-1-2{)}^2}=\sqrt{16+9}=5$ and HD = |7 - 2| = 5. Every post is 5 km away, so all four lie on the circle of radius 5 km centred at $H$, and no post is nearer than another. The negative coordinates in $B$ and $C$ do not shorten those distances, because the differences are squared.