what is the volume of .540 m NaOH solution that contains 15.5 g of NaOH?

Find the number of moles of given NaOH:
15.5 g NaOH * (1 mole NaOH / 39.9971 g NaOH)
= 0.388 moles NaOH

Now put the numbers into this formula:
molarity = moles NaOH / L
0.540 M = 0.388 moles NaOH / L
= 0.718 L