Assuming you want to find the pOH of an aqueous Sr(OH)2 solution.
1st: Find the concentration of OH- in the solution
[OH-] = 2[Sr(OH)2]
= 2(0.27) M
= 0.54 M
(the number of OH- ions released by H2O molecules are negligible)
2nd: find p[OH-]
you have to know that
pOH = -log [OH-]
= -log (0.54)
= 0.268 (to 3 significant figures)
and that's it.