(1 + ay)^n, solve for a, n

[Clifford]

Given (1 + ay)^n, the first three terms are 1, 12y and 68y^2 respectively. Solve for a and n.

I have absolutely no idea where to begin this question. Can somebody put me on the right track?

 

[pka]

\(\displaystyle \L
\left( {x + y} \right)^n = \sum\limits_{k = 0}^n {\left( {\begin{array}{c}
n \\
k \\
\end{array}} \right)x^{n - k} y^k } = x^n + nx^{n - 1} y + \frac{{n\left( {n - 1} \right)}}{2}x^{n - 2} y^2 \cdots\)