To find if three number are a triangle, the formula is A^2 + B^2 = C^2. This means that when you square the lowest two terms they need to add to the square of the third number.
12^2 = 144
13^2 = 169
144 + 169 = 313
26^2 = 676
Since 313 does not equal 676 these numbers cant be the sides of a triangle.
