how to solve a multinomial theorem

find the no. of solution of r1 + r2 ......r(s)=n
for the expansion.... (a1 + a2 + ...... a(s) ) ^n
then find particular values of r1,r2...r(s)
and then put it in the general term
= n! [   (a1)^r1  .  (a2)^r2  . ..... a(s)^r(s)   ]/ (r1)!.(r2)!....r(s)!
its a bit tough to explain...so please do follow the concept.

you need to just use the basic concepts of binomial theorem