Number of moles = Mass / Molar mass
Molar mass of NaOH=40 g/mol
By Using the definition of molarity to find the volume of the solution:
Molarity= Volume of solution (in liters)Number of moles of solute
M=Vsol( in L)nNaOH⇒3=V(84/40)⇒V=0.7L=7×10−1L