Mass of solution = 2.5 Kg

Molality = 0.25 m

Molar mass of urea (NH2CONH2) =14 + 2 × 1 + 12 + 16 + 14 + 2 × 1 = 60 g mol−1

Let mass of urea (NH2CONH2) = w gram

Number of moles of urea  = w / 60 = 0.01667 mol

Mass of urea in Kg = w/1000  = 0.001w kg

Mass  of solvent = total mass  - mass  of solute  = 2.5  - 0.001 w kg

so,

0.25 = 0.01667 w/( 2.5- 0.001w)

w= 37 g of urea.