Expansion of (1 + ay)^n: first 3 terms are 1, 12y, 68y^2

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+...

Can someone please help me with this problem I don't even know where to start!

Use the given info and solve the system of two equations which results.

\(\displaystyle \L\\a\cdot{n}=12\)

\(\displaystyle \L\\\frac{n(n-1)}{2}\cdot{a^{2}}=68\)

so like do elimination or substitution with those equations?