What’s the iupac name for ch2=c(ch2ch3)2?

            The IUPAC name of given compound is 2-Ethyl-1-Butene.

Explanation:
                   First of all, a longest chain containing a double bond was selected as a parent chain, as it has four carbon atoms and a double bond at position 1, so parent name is,
                                               1-Butene

Ethyle (CH₃-CH₂-) is the substituent present at position 2. So, complete name will be as follow,

                                         2-Ethyl-1-Butene

can also be written as,

                                         2-Ethylbut-1-ene