Pls help with ph of strong bases

Hi!
In the topic of acids and bases in a level aqa chem, I have a question about finding out the concentration of strong bases using the ph
Why do we divide the [OH-] by 2 when finding out the [Ba(OH)^2]? I thought we would have to multiply by 2 because when the base/alkali dissociates, aren’t two molecules of hydroxide ions released? I’m so confused pls help if you know, tyy!

Hey!

Try writing out the equation of what is happening when Ba(OH)2 dissociates and have a look at the stoichiometry between the base and hydroxide ions