The term 'City on the Mexican Border' is referring to the city of Laredo, which has 6 letters. Laredo is a city located in the state of Texas, United States, and is situated on the banks of the Rio Grande river, which forms the border between Mexico and the United States. It is the largest inland port in the United States and is a major hub for international trade. The city is known for its vibrant culture, with a mix of Mexican and American influences.