note the squares...
13 is possible because it is a sum of two squares (integers) 9 + 4 .. use (0,3) & (2,0) ... get the square roots and place on different coordinate axes... for a simpler value.
but there is none for 15...
there is one for 18 ...
18 = 3^2 + 3^2 ... try (0,3) ,(3,0) , (3,6), (6,3).
