126 units of NBr3 react with 3/2 x 126 = 189 units of NaOH. There will be 3 units of NaOH unreacted.
189 formula units of NaBr will be formed.
The term "formula units" is new to me. I assumed that "formula units" means the same thing as moles, or any unit of mass divided by molecular weight.
Formula unit is essentially the same as molecule. Formula units come about because some don't like to call NaCl a mole or even NaCl as the molecular formula. Why. Because NaxClx is a better way of representing how Na and Cl2 combine to form NaxClx. There are no discrete NaCl molecules as in CO2, CS2, O2, etc. So they get around that problem by call NaCl a formula unit instead of a molecular unit or molecule or mole. CO2 is a formula unit as well as a molecular unit but NaCl is not both. Personally, I think it's a waste of time to try and be as picky as that. By the same reasoning, I would just as soon stick to molecular weight (instead of molar mass), weight instead of mass, etc etc. I think we confuse the students when we try to get too technical. Chemistry is already too technical for many students at the beginning level. Everyone KNOWS we mean mass when we say weight. We can become more picky at advanced levels. But alas, the trend is to be more picky early in the game.