Let us first draw the structure of 2,4 - Dimethylhexane.

The name of compound depicts that the parent chain is of 6 carbon atoms as it is hexane and consists of methyl group on the second and fourth carbon. Hence the structure is given as:

In 1,4- Dimethylbenzene, benzene consists of two methyl groups, one at first carbon and the other at the fourth. Hence the structure is given as,

In Pent-3-en-2-ol, the parent chain consists of 5 carbon atoms. It has an alcohol group (-OH) at its second carbon and a double bond at the third. The structure is thus derived as,