You can find the number of mol in 2.3g of NaCl molar mass (58.4g/mol) which gives you 0.039 mol. Then you can convert 250 ml into L which gives you 0,250L. Now you simply put it in the formula (b= m(NaCl)/V(H2O)) it gives you b=0.039/0.250= 0.158 mol/L.
