Question

How many moles of NaOH are required to prepare 2.90 L of 1.8 M NaOH?

Answer 1

Here is a couple of useful formula to use when you get problems like this.

$Molarity= \frac{moles}{Liters}$        ----->      $moles= (Molarity) x (Liters)$

since they are asking for moles, we are using the one on the right. 

Molarity= 1.8 M
Liters= 2.90 L

moles= (1.8 M) x (2.90 L)= 5.2 moles of NaOH

Answer 2

V=2.90 L
c=1.8 mol/L

n(NaOH)=vc

n(NaOH)=2.90L*1.8mol/L=5.22 mol