In fact I believe you
have to.
Brego is a required action (you don't have a choice) whereas
City of the Trees is an optional action (it says "you may...") and required actions
always take place before optional actions.
If they both happened at the same time (say, both required actions) then the Free Peoples player gets the choice what happens in which order.
ASIDE: This is why the
![Raider [Raider]](https://lotrtcgdb.com/forums/Smileys/classic/raider.png)
cards like
Desert Soldier were changed from required to optional because if they were required (like ambush) the FP player could always choose to have the removing
![3 [3]](https://lotrtcgdb.com/forums/Smileys/classic/3.png)
ability resolve
before the ambush.
Thranduil