The final reaction needs to have ClO on the product side of the equation. We have to reverse the first equation, which will also change the sign of dH. (+122.8 kj/mol)
The next equation can remain as is because our final step calls for O2 as a product which we already have. Making dH: (-285.3 kj/mol)
Add them: +122.8 kj/mol + (-285.3 kj/mol)= -162.5 kj/mol
