So here we need to determine a systematic name of the following alkane. Here our carbon chain is this portion, here because it's composed of 12345678 carbons. Remember as long as our carbon chain has more than six carbons it becomes apparent chain and benzene becomes a substituent and then besides that benzene we have these two carbon substituents which are methyl groups.
Now remember we number from the end closest to a substituent. We numbered this end as one because on carbon number one we immediately have a substituent in the form of benzene. So 12345678 and eight carbon chain is octane. Benzene as a substituent is fennel, and then we have methyl groups on carbon 5.
Now we name substituents alphabetically. M comes before P, so those two methyl groups are on five. So we have to list 5 twice because you have. If you just said 5 dimethyl, all I would know is that one of the methyl groups is on carbon 5, but where is the other one? So we have to say 5-5 to say that both of them are on that carbon 5.
S 55. There's two identical substituents, so use the numerical prefix die dimethyl 1 phenyl octane. So this will be the name of my alkane compound.