You can see that the longest chain is the one that is horizontal (It's not in the ramifications). And we have to enumerate from right to left the number of carbons because the first functional group is nearest to the right. This functional group is in the fifth carbon and it's called 'sec-butyl'. Let's see better by doing the illustration:
The order of the full name would be 'x-functional group ´principal chain prefix´ane'. The prefix of the chain is 'deca' because this prefix indicates that we have 10 carbons in the principal chain, so the full name of the compound would be: 5-sec-butyldecane.