The last line should be multiplied by .

By the simplest form of the binomial theorem we have

(1)

Also, by the last formula in this Wiki section, we have

(2)

We have that comes from the first factor , so the rest of the expression must contribute towards . We can get from (1) and from (2); from (1) and from (2); and from (1) and from (2). The product of the corresponding coefficients make the three terms in the expression