From the dilution law

  C\(_1\)V\(_1\) = C\(_2\)V\(_2\)

  C\(_1\) = Initial Concentration of NaOH = 0.6M

  V\(_1\) = Initial Volume of NaOH = 300cm3

  C\(_2\) = Final Concentration of NaOH = 0.4M

  V\(_2\) = Final Volume of NaOH = ?

  V\(_2\) = \(\frac{C_1V_1}{C_2}\)

  V\(_2\) = \(\frac{0.6 \times 300}{0.4}\)

  V\(_2\) = 450 cm\(^3\)

Correct Computation as it follows the equation specified