Find moles of HCl, and thus the moles of H+ ions.
Find moles of Sr(OH)2, and thus the moles of OH- ions which would be twice the amount as (OH)2.
Then find which reactant is in excess (has most moles). The moles of H+ or OH- that don’t react (neutralise) will cause the pH to change.
Convert this value into a concentration by dividing by the total volume of both solutions, and into dm^3. This will give you [OH-] or [H+],
Then it’s easy to work out pH. If you have [OH-] excess, then take pH = 14 - pOH, if you have [H+] then pH = -log[H+].