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

Apparent weight (after object is submerged in the fluid) is equal to the force on it due to gravity minus the buoyant force.
F_apparent = F_gravity - F_buoyant

Therefore,
F_buoyant = F_gravity - F_apparent

In this case, F_gravity = 26N, and F_apparent = 11N, so

F_buoyant = 26N - 11N = 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.