<p>I'm having trouble finding the coefficient of x^3 in (1+11x)^9 . I know you use factorials but I'm not really grasping how. Thanks in advance cx</p>
<p>The x^3 term is (9C3)(11x)^3 (1)^6 = 84*(11x)^3 = 111804x^3, so the x^3 coefficient is 111804.</p>
<p>(Here 9C3 = 9 \choose 3 = 9!/(3!*6!)).</p>