how many grams of sucrose must be dissolved in 1.5kg of water ot produce a solution which has a freezing point of -2.40 degrees C?

delta T = Kf*molality

Substitute and solve for m.

m = mols/kg solvent.
Substitute and solve for mols.

mols = grams/molar mass. You have molar mass and mols, solve for grams.