... explanation ?
Okay first look at the alkene. How many carbons are there ? 4, right ? So that means it is butene OR Butane.
BUT it has a carbon double bond so it must be an ALKENE hence it is BUTENE !!
Now the isomer of butene can be but-2-ene. The numeral 2 signifies that the carbon double bond is located on the SECOND CARBON.
So thats how you do it