A way of solving this question is using the molarity formula and finding the number of moles present in the solution, the molarity formula is:
M = n/V
where:
M = molar concentration in mol/L
n = number of moles
V = volume in liters
Now let's add the values to the formula
0.50 = n/0.750 L
n = 0.375 moles
Now let's use the molarity formula again, but with the new volume (750 mL - 150 mL = 600 mL)
M = 0.375/0.600
M = 0.625 mol/L, this is the new concentration of NaCl