∫▒〖1/(3-9x^2 ) dx〗

i know this problem is not hard but im not getting what the back of the book is getting.

the directions told me to use this formula:

∫▒〖du/(a^2-u^2 ) 〗= 1/2a ln (a+u)/(a-u)+ C

note: there should be absolute bars around the ln quantity i dont know how to make those on my computer. this problem seems easy enough. simple substitution and then solve it, but im not getting it. any help would be appreciated. thanks in advance.