a. The structural formula of 3-ethyl-2-methylhexanecan be derived as follows: -

1. At first we have to find the parent chain. Here the parent chain is hexane. Hence we draw a hexane first : -

2. Then in the next step we find the position of the substituent and the substituent attached to the parent chain. Her, we have 2 substituents one ethyl group at 3^rd position and one methyl group at 2^nd position. Adding them stepwise we get after adding methyl group : -

After adding the ethyl group we get the final molecule as : -

b. The structural formula of But-2-ene can be elucidated as follows: -

1. At first we have to find the parent chain which in this case is a 4 membered carbon ring so let us first draw a butane:

Now from the name But-2-ene we come to know that it is an alkene with a double bond in between 2^nd and 3^rd carbon. Hence, we derived at the structure of But-2-ene as shown : -