Question 970463
binomial expansion

1
1  2 1
1 3 3 1
1;;   4;;  6 ;; 4; 1
1; 5; 10;; 10 ;;5;; 1
1 ;;6;; 15;; 20;; 15;; 6 ;;1
1 ;;7 ;;21;; 35;; 35;;  21;; 7;;  1

(2x) is the leading term
The first term is (2x)^7
The second term is 7 (2x)^6
The x^3 term is the fifth number in the sequence
it is 35 (2x)^3

Remember that the 2 has to be cubed as well
So the term involving x^3 is 280 x^3