If you give first strike to a creature with last strike, it’ll deal combat damage twice: once during first-strike combat damage and again during last-strike combat damage. If you give a creature double strike to a creature with last strike, it’ll deal combat damage three times: once during first-strike combat damage, again during normal combat damage, and again during last-strike damage. Essentially, double strike + last strike = triple strike.

Last strike is opposite of first strike. A creature with last strike deals combat damage after when most creatures deal combat damage. A creature with last strike deals combat damage only if it survives both first-strike and normal combat damage.