start by listing all the factors for one of them, 18 since its easier
2 (2 x 9)
3 (3 x 6)
6 (6 x 3)
9 (9x2)
18 (18x2)
2 is your greatest common factor because 130 cant go into anything other than 2 evenly, try it divide 130 by 3, 6, 9, 18 their all decimil answers.
2 is your answer