13ab - 9cd - xy + 5xy + 15cd - 7ab + 6xy - 3cd
Arrange the polynomial with like terms together and combine them.
= (13ab - 7ab) + (-9cd + 15cd - 3cd) + (-xy + 5xy + 6xy)
= 6ab + 3cd + 10xy
Hence, the addition of 13ab - 9cd - xy, 5xy, 15cd - 7ab, 6xy - 3cd is 6ab + 3cd + 10xy.