The first three terms in the expansion of (1 + ay)^n are 1, 12y and 68y^2.

Evaluate a and n. Use the fact that:

(1+ay)^n = 1 + nay + {[n(n-1)]/2}(ay)^2 +....

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This is as far as I've gotten and I'm not sure what I should do next:

nay=2y

=> na=12.

=> n=12/a

=> a =12/n

{[n(n-1)]/2} (ay)^2 = 68 y^2.

{[n(n-1)]/2} (a)^2 = 68

{[(n^2) - n]/2} (a)^2 = 68

[(n^2) - n](a)^2 = 136