Calculate the pOH of an aqueous solution of 146.6 ml of 0.000625 M HBr(aq) after dilution to 850 ml.
(HBr) diluted = 0.000625 x (146.6/860) = about 1.08E-4
pH = -log(H^+) = -log(HBr) since HBr is a strong acid and ionizes 100%.
Then pH + pOH = pKw = 14. You know pH and pKw solve for pOH.