They are asking you for OH- ions concentration, which is why you have to find the pOH of the solution. You can easily do this with this formula:
pH + pOH = 14
Solve for pOH and you get:
pOH = 14 - pH
Replace the pH with it's value:
pOH = 14 - 11.8
pOH = 2.2
Now that you have the pOH you can find the concentration of OH- ions with this formula:
pOH = -log[OH-]
Solve the formula for [OH-] and you get:
[OH-] = 10^(-pOH)
Now replace pOH with it's value:
[OH-] = 10^(-2.2)
[OH-] = 0.006309 M
The concentration of OH- atoms is 0.006309M or in the scientific notation 6.309×10^(-3)M
