Help with summation question? I have a problem involving sums:

I just have no idea how to solve this.
I know how to solve sums but this does not make sense to me. I need the value of the 'a' but in the first one, 'a' must be 9/8 and it must be something else in the second, so forth...
So I cannot solve it...
How do I go about solving this?
Thanks 

Hint: 
$$(1+a_i)^3=1+3a_i+3a_i^2+a_i^3.$$
