The coefficient of x2y3z4 in (ax−by+cz)9 is

In the expansion of (ax−by+cz)9 , the term containing z4 is 9C4(ax-by)5(cz)4  In the expansion of (ax−by)5, the term containing y3 is 5C3(ax)2(−by)3∴ The coefficient of x2y3z4 is 9C4 5C3a2(−b)3(c)4= 9C4 5C3a2(−b)3(c)49×8×7×624×10×a2b3c4=−1260a2b3c4