Which court case in 2013 struck down the formula for determine the states covered under the section 5 preclearance requirement of the voting rights act of 1965?

That would be the Shelby County, Alabama v. Holder case from 2013.

In this court case the Supreme Court ruled 5-4 in favor of striking down the the part of the 1965 Voting Rights Act. This ruling freed and thus allowed 9 mostly southern states to change their election laws. They do not have to get a federal approval beforehand anymore.