Abc has side lenths 20, 24, and 31. do the side lenghts form a pythagorean triple?

No they don't.
Phythagorean triple is when the sum of the squares of two numbers is equal to the square of another number.

20^2 + 24^2 = 400 + 576 = 976
31^2 = 961

20^2 + 24^2 should have been equal to 31^2 but that's not the case. So, the side's length don't form a triple.

Hope it helped!