Answer:
[tex]= > \ ^9C_6\ a^3\ b^6[/tex]
Step-by-step explanation:
[tex]= > T_{r+1}=\ ^n{C_r}\ x^{n-r} \ y^r[/tex]
[tex]= > T_{r+1}=\ ^9C_r\ (ax^4)^{9-r}\ (-bx)^r[/tex]
[tex]= > T_{r+1}=\ ^9C_{r}\ a^{9-r}\ (-b)^r\ x^{36-4r}.x^r[/tex]
[tex]= > T_{r+1}=\ ^9C_{r}a^{9-r}(-b)^r.x^{36-3r}[/tex]
[tex]We\:\ need\:\ coefficient\:\ of\:\ x^{18}[/tex]
=> 36 - 3r = 18
=>3r = 18
=> r = 6
Therefore Coefficient is
[tex]= > \ ^9C_6\ a^3\ b^6[/tex]