The third, fourth, and fifth terms in the expansion of (x + a)^n are 84, 280, and 560, respectively. Find n.
When x=2, then:
(2 + a)^7 =x^7 + 14 x^6 + 84 x^5 + 280 x^4 + 560 x^3 + 672 x^2 + 448 x + 128
