The term (m+2)(m+1)am+2xm
for m = 0 from the first sum equals 2a2.
Then the term −
(m+1)am+1xm for m = 0 from the second
sum equals − a1.
Note that the third sum does not contain constant terms (m is
at least 1). Thus the constant term in the whole expression is 2a2.−