Crowd's Favor is a common Instant card from Magic 2015 (M15), collector number 138. Current market price: $0.20 (foil $0.55), TCGplayer market prices via Scryfall.
Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for {1} or one mana of that creature's color.) Target creature gets +1/+0 and gains first strike until end of turn. (It deals combat damage before creatures without first strike.)