When do you use Hfus? Why can’t we use q=mcΔt at this point?

when there is heat of fusion Hfus we can't use q=mcΔt because there Is a change of state from solid to liquid with out a corresponding change in temperature (hidden heat).

Therefore whenever there is a change from one state to another there is no change in temperature

So instead of using q=mcΔt

We use q=m*Hfus

What is specific Latent heat of fusion?

Specific Latent heat of fusion of a substance is the quantity of heat required to change a kilogram of a substance from the solid state to The liquid state at melting point.

The S. I unit is in joules per kilogram (j/kg)

93.2 ml + 3920 ml = 4013 ml total  

(93.2 ml) * (2.03 m) / (4013 ml) = 0.0471 m

the volume in in l of the diluted solution would be 0.0471 m.