use a bit of logic
the longest side of a triangle has to be less than the sum of the legnths of the other two sides (or else they won't connect or you get a striahge line or something)
baseically
longestside<otherside+shortestside
12,13,5
13<12+5 is that true?
13<17
true
it can be a triangle
6.7,6.5,12.6
12.6<6.7+6.5
12.6<13.2
true, can be a triangle
13,12,11
13<12+11
13<23
true
can be a triangle
13,4,6
13<4+6
13<10
false, cannot be a triangle
all of them can be legnths of a triangle except last one