In the equation y = 3x - 1 what is the value of y when x equals -3?

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.

I’m really confused and need your assist me with this question i’ve never been taught

Which situation is an example of an observational study?