The solution can be considered as a neutral solution. In order to determine its pH we should get the concentration of OH- in the solution. 

We first let a 1 liter of solution of 10^-9 M NaOH.

In a liter of solution there are 10^-9 moles NaOH thus there are 10^-9 moles OH- (Since moles NaOH = moles Na+ = moles OH-).

Next, remember that neutral water has 10^-7 moles OH or H+ right? So we add the moles of NaOH with the moles of OH derived from water.

10^-9 + 10^-7 = 1.01x10^-7 moles NaOH

pOH = -log (OH-)

pOH = -log (1.01x10^-7)

pOH = 6.99

pH = 14 - pOH

pH = 14 – 6.996

pH = 7.004 ~ 7.00 therefore neutral.