Answer:
C(n, 2)a^(n-2)b^2
Step-by-step explanation:
Generally, the expansion is written in decreasing powers of "a", so the first few terms would have variable constellations that look like ...
a^n, a^(n-1)b, a^(n-2)b^2, ...
The coefficients would be (in order), C(n, k) for k increasing from 0, so the coefficient of the 3rd term would be C(n, 2).
