America is a continent so technically there is no capital. This of course is unless you are in Australia. Like most continents the Americas have an organisation for regional cooperation and solidarity. In the Americas it is "The Organization of American States". It was founded in 1948 and currently 35 countries are members of the OAS.
The head office of the organisation is in Washington. If you define "capital" by where the seat of the organisation is located, Washington would be the capital of America. It of course is also the capital of the United States of America; especially countries from South America would therefore disagree with this definition.
For the purpose of this exercise we still want to define Washington as the capital of America although we do want to mention that fact that there is not one capital for the entire continent.