The difference here is that the centre of
mass is now a distance L/3 away from the edge
of the shelf rather than a distance of L/2
as it was in the previous part of the question.
In addition to this weight, there are still
two normal reaction forces due to the brackets
acting upwards on the shelf. For it to be
in equilibrium, the total of all these forces
must be equal to zero. The total of all the
moments acting on the shelf, about any point,
must also be equal to zero.
