({4} magic symbol) ({U} magic symbol) ({U} magic symbol) ({U} magic symbol) ({R} magic symbol) ({R} magic symbol) plus enough mana to cast the additional instant card available.
Steps
Cast
Bonus Round by paying ({1} magic symbol) ({R} magic symbol) ({R} magic symbol) , causing you to copy any instant and/or sorcery spell you cast this turn.
Cast any other instant card from your hand by paying its mana cost.
Bonus Round triggers, copying the instant.
Holding priority, cast
Spelljack by paying ({3} magic symbol) ({U} magic symbol) ({U} magic symbol) ({U} magic symbol) , targeting the instant.
Bonus Round triggers, creating a copy of
Spelljack that is targeting the original
Spelljack.
Resolve the copy of
Spelljack, countering the original
Spelljack and exiling it with the ability to cast it for free from exile.
Cast
Spelljack from exile without paying its mana cost, targeting the instant cast in step 1.