7,12,19

19^2-12^2 = (19-12)(19+12) = 7\cdot 31=217 \ne7^2

Therefore,

7,12,19 does not constitute a Pythagorean triplet

5,12,13

13^2-12^2 = (13-12)(13+12) = 25 = 5^2

Therefore,

5,12,13 forms a Pythagorean triplet

11,12,15

15^2-12^2 = (15-12)(15+12) = 3\cdot 25 = 75\ne 11^2

11,12,15 does not constitute a Pythagorean triplet

11,12,18

18^2-12^2=(18-12)(18+12) = 6\cdot30 = 180\ne11^2

11,12,18 does not constitute a Pythagorean triplet