As the problem states, with strong bases<em> the hydroxide ion concentration [OH⁻] is equal to the concentration of the base</em>. That means that in this case:
[OH⁻] = 0.0001 M
With [OH⁻] we can <u>calculate the pOH of the solution</u>:
pOH = -log[OH⁻]
pOH = 4
Then with the pOH, we <u>calculate the pH of the solution</u>: