If I attach an equip in unit, and oponent react killing ruined rex, the deathknell is resolves before unit is attached or after?

Deathknell resolves first, and in Riftbound the Equip never attaches at all.

Your Equip ability goes on the chain, then your opponent's kill is put on top of it. LIFO resolution means the kill happens first — and per Rule 428.1.a.1.b the Deathknell trigger is added to the chain as a pending item before Ruined Rex is put in the trash, so it lands above your Equip ability. The 4 damage therefore resolves while Rex is already gone and nothing is attached to him yet.

After that, the Equip ability resolves and simply whiffs: Rex is in the trash, so the chosen target is no longer legal and the Attach instruction does nothing (Rule 359.3.e.12). The gear stays at your base — unattached — and the Equip cost is still spent, no refund.

If the order had been reversed (Equip resolved first), the gear would attach and get the bonuses, but the Deathknell would still resolve after the kill, and the orphaned gear would be recalled during the next cleanup.