Answer:
5.12 moles of NaCl are required to prepare 0.80 L of 6.4 M NaCl.
Step-by-step explanation:
 Molarity (M) is a concentration measure that indicates the number of moles of solute that are dissolved in a given volume. 
 The Molarity is then determined by: 
 

Molarity is expressed in units
.
6.4 M NaCl indicates that 1 liter of solution there are 6.4 moles of NaCl. 
You can apply a rule of three as follows: if in 1 liter of solution there are 6.4 moles of NaCl, in 0.8 L how many moles are there?

moles=5.12 
5.12 moles of NaCl are required to prepare 0.80 L of 6.4 M NaCl.