State if three numbers can be the measures of the sides of a triangle 11, 6, 13

Please help!

Every time you drive you will experience a combination of driving conditions and environments.

Which three lengths could be the lengths of the sides of a triangle? 21 cm, 7 cm, 7 cm 9 cm, 15 cm, 22 cm 13 cm, 5 cm, 18 cm 8 cm, 23 cm, 11 cm

1) prove that 731^3−631^3 is divisible by 100 2) prove that 99^3−74^3 is divisible by 25