In naming this 4-carbon alkene first we have to locate the double bond. It is in the second carbon and therefore it is called 2-butene or but-2-ene. Next, we consider the substituent in the molecule. We can notice that there is a Br and a Cl attached to the second and third carbon. Alphabetically, Bromine should be named first and therefore the carbon where Br is attached is named as the Carbon 2; carbon 3 is not the carbon where Cl is attached.
So we can partially name it as 2-bromo-3-chloro-but-2-ene.
Now it is up to you whether it is a cis or a trans butene.