Please help me on this please it’s due right now

Trent wants to buy 2 packs of trading cards for 3 dollars each. the trading card packs that trent normally buys tend to come in packs of 6, 10, 12, or 15 cards. after selecting 2 packs, trent found that the first pack of cards cost 25 cents per card, and the second pack cost 30 cents per card. trent uses this information to write the equations below in order to compare c, the number of cards in each pack.

Find the solution of this system of equation -7x+y=-20 9x-3y=36

Erin bought christmas cards for $2 each to send to her family and friends. which variable is the dependent variable?