what is the largest city where the population is 6620100?
{'header': ['Province', 'Provincial capital', 'Largest city', 'Area (km 2 )', 'Population (2013)'], 'page_title': 'South Africa', 'section_title': 'Provinces', 'caption': 'Provinces', 'rows': [['Eastern Cape', 'Bhisho', 'Port Elizabeth', '168966', '6620100'], ['Free State', 'Bloemfontein', 'Bloemfontein', '129825', '2753200'], ['Gauteng', 'Johannesburg', 'Johannesburg', '18178', '12728400'], ['KwaZulu-Natal', 'Pietermaritzburg', 'Durban', '94361', '10456900'], ['Limpopo', 'Polokwane', 'Polokwane', '125754', '5518000'], ['Mpumalanga', 'Nelspruit', 'Nelspruit', '76495', '4128000'], ['North West', 'Mahikeng', 'Rustenburg', '104882', '3597600'], ['Northern Cape', 'Kimberley', 'Kimberley', '372889', '1162900']]}