One mole of carbon weighs 12 grams and one mole of hydrogen weighs 1 gram. Therefore, one mole of CH2 weighs 12 + 1 + 1 = 14 grams.
56 divided by 14 = 4
Therefore, you need 4 "moles" of CH2 to equal 56 grams of the unknown substance. To get the equivalent of 4 "moles" of CH2 you need one mole of C4H8.
Hope this helps :)