Question

A brick weighs 26 N. Measured underwater, it weighs 11 N. What is the amount of buoyant force exerted by the water on the brick?

Answer 1

buoyant force = weight - apparent weight = 26 - 11 = 15N

Answer 2

Buoyant force is equal to upthrust therefore 
Upthrust = weight of brick in air - weight of brick in water 
Upthrust = 26 - 11
Upthrust =15N 
It's S.I unit is Newton (N) because it is a force