Ca(OH)₂ ==> Ca²⁺ + 2 OH-
Ca(OH)₂ is strong Bases
Therefore, the [OH-] equals 5 x 10⁻⁴ M. For every Ca(OH)₂ you produce 2 OH⁻.
pOH = - log[ OH⁻]
pOH = - log [ 5 x 10⁻⁴ ]
pOH = 3.30
pH + pOH = 14
pH + 3.30 = 14
pH = 14 - 3.30
pH = 10.7
hope this helps!