a³ + b³ + c³ + 3abc = ( a + b + c)(a² + b² + c² + ab + bc + ca)
if a + b + c = 0. then a³ + b³ + c³ = 3abc
here a + b + c = 15 + (-9) + (-6)
a + b + c = 15 - 15
a + b + c = 0
so a³ + b³ + c³ = 3abc
15³ + (-9)³ + (-6)³ = 3(15)(-9)(-6)
= 2430
so the answer is 2430
