Start by dividing 42 by 1, then 2, then 3, and so on. Only include the ones that produce a whole number.
42 / 1 = 42
42 / 2 = 21
42 / 3 = 14
42 / 6 = 7
42 / 7 = 6 (appears again, so you can stop the process now)
42 = 1 x 42
42 = 2 x 21
42 = 3 x 14
42 = 6 x 7
The factors of 42 are 1, 2, 3, 6, 7, 14, 21, and 42.