ते to you, दशरथः Dasaratha, कश्चित् न is nobody, त्वं च you also, तस्य to him, कश्चन न nobody, सः राजा that king, अन्यः another person, त्वम् you, अन्यः another person, तस्मात् therefore, यत् what, उच्यते is being told, कुरु do.

Dasaratha was nobody to you and you are nobody to him. That king is one person and you are another. Therefore, do what I tell you. (It does not matter what you are to him and what he means to you).