Help with general binomial expansion

Find and simplify the term containing a^4 in (a+2b^9.

Step by step please

Note: here (r+1) is in subscript.

let t(r+1) be the general term of the expansion of (a+2b)^9

t(r+1) = C(9,r).a^(9-r).(2b)^r --- eqn (1)

For term containing a^4,
a^(9-r) = a^4
equating powers,
9 - r = 4
r = 5

Putting r = 5 in eqn (1)

t(5+1) = C(9,5).a^(9-5).(2b)^5
t6 = 126.a^4.(2b)^5 = 126.32.b^5.a^4 = 4032.b^5.a^4

the term containing a^4 is 4032.b^5.a^4