Answer
$\Delta S_R' = -0.723 J/K$
Work Step by Step
From (c), the change in entropy of the block R is $\Delta S_R = +0.723 J/K$
Note that block R receives heat from reservoir. This means the magnitude of change of entropy in the block is the same as the reservoir. The positive sign indicates that energy is being absorbed from the reservoir.
So, for the reservoir, the heat is released into the block, so the sign will be negative as an indication of released heat.
$\Delta S_R' = -0.723 J/K$
