How do you find the reaction at a pivot when a plank is in equilibrium?

If the plank is in equilibrium, then it's not moving.

If it's not moving, then the forces on it must add up to zero ... otherwise it would

be accelerating.

At the pivot, there are two forces: the weight of the plank is directed down, and

the reaction is directed up.

The two forces must add up to zero.

Can you take it from there ?