I've been given the first three terms in the expansion of (1+ay)^n:

1, 12y, and 68y^2.

I'm asked to evaluateaandnusing the fact that:

(1+ay)^n= 1 +nay + n(n-1)/2(ay)^2 +.....

Any help in this problem would be appreciated, what is meant byevaluate?