According to IUPAC rules first thing you need to see is longest carbon chain which is 10(deca) over here.
Then while numbering the Carbon double bond is given preference.
So above compound is 10 carbon chain in which 3rd carbon is double bonded.
Thus, 3-methylenedecane is correct name.
Few more such examples to make it more clear:
Example Source: ChemSink
Basic IUPAC rules:
Find the longest chain in the molecule and name it.
Name all groups attached to the chain as alkyl substituents.
Number the carbons of the longest chain beginning with the end that is closest to a substituent.
Write the name of the alkane by first arranging all of the substituents in numerical order (each preceded by the number of the carbon to which it is attached and a hyphen), and then add the name of the stem.
The halogen is treated as an alkyl substituent, without any priority.
Ethers are named as alkoxyalkanes. The first carbon of the alkoxy substituent is numbered as C-1 of that substituent.
Note that these rules are changed as of 2008. Older HW and exams have slightly different answers.
Rules Source: http://valhalla.chem.udel.edu/nomenclature.html