The term (m+2)(m+1)a_{m+2}x^{m}
for m = 0 from the first sum equals 2a_{2.}

Then the term −
(m+1)a_{m+1}x^{m }for m = 0 from the second
sum equals − a_{1.
}Note that the third sum does not contain constant terms (m is
at least 1). Thus the constant term in the whole expression is 2a_{2.}−
a_{1. }