these ones it helps to draw. you have four carbons and each one must have 4 covalent bonds coming off of it, but the double bond will act as a 2 covalent bonds between carbon atoms rather than 1. You should get C4H8. Alternatively the alkene formula can be used (alkenes are molecules with a C to C double bond) which states CnH2n will be the Molecular formula where n is the number of carbons.