Hi guys I have an exercise I don't know how to approach, would be cool if you could give me a tip or two!  
 A sequence $a\_{n}$ is defined by a dependency :  
 $$ \sum\_{i, j, k \geq 0}^{i+j+k = n } a\_ia\_ja\_k = (n+1)^{\overline{5}} \text { 
 for $n\geq$ 0}$$
I am to solve the given summation and find an equation for $a\_{n}$.
Thanks in advance!