What mass of NaCl must be dissolved in 65 grams of water to lower freezing point by 7.50C

mass of nacl =  W2 = ?
Tf =Kf x W2 x 1000 / m2 x W1 
given:
i = vant hoff factor of nacl = 1.87

W1 = 65.0 g

M2 = 58.5 g /mol

Kf = 1.86 C/m

Tf =7.5⁰C

W2 = Kf x W2 x 1000 / M2 x W1

​ therefore , W2 = 28.67 g