It appears that ALL of the bath water is frozen and not just a part of it.
heat lost by rocks + heat absorbed by
ice melting + heat absorbed by liquid water to raise T to final T = 0
heat lost by rocks is
mass rocks x specific heat rocks x (Tfinal-Tinitial)
heat absorbed by ice to melt it.
mass ice x delta Hfusion
heat to raise temperature of liquid water.
mass water x specific heat H2O x (Tfinal-Tinitial)
Just add all of these in a string and solve for mass rocks. In a string it looks like this
[mass rocks x sp.h. rocks x (Tf-Ti)] + (mass ice x delta Hfusion) + [mass melted ice x sp.h H2O x (Tf-Ti)] = 0
Substitute and solve for mass rocks. The final answer is a large number.