Răspuns:
2009:[(13×a+47)×3-10]=7
[(13xa+47)X3-10]=2009:7
(13xa+47)x3-10=287
(13xa+47)X3=287+10
(13xa+47)X3=297
(13xa+47)=297:3
13xa+47=99
13xa=99-47
13xa=52
a=52:13
a=4
