You can cast a card with jump-start from your graveyard by paying all its regular costs and one additional cost: discarding a card from your hand. Casting a spell with jump-start follows all the normal timing rules, so sorceries with jump-start are still limited to your main phases. A spell with jump-start that was cast from your graveyard can still be countered, and if it has targets, it won't do anything if all its targets disappear or otherwise become illegal. After a spell with jump-start cast from your graveyard resolves, is countered, or leaves the stack in any way, it's exiled.

Mentor is an ability that triggers whenever the creature with mentor attacks. You choose another attacking creature with lesser power as the target. As the ability resolves, compare the powers of the two creatures again. If the target creature's power is still less than the power of the creature with mentor, put a +1/+1 counter on the target creature.

The detectives of Ravnica have plenty of choices to make, and so will you when you're casting split cards! Each split card has two card faces on it, and each card face represents an individual spell you can cast.