Why do you think triangular structures are so stable?

Which is an equation for the nth terms of the sequence 12,15,18,21

Jackie purchased 3 bottles of water and 2 cups of coffee for a family for $7.35. brian bought 4 bottles of water and 1 cup of coffee for his family for $7.15. how much does each bottle of water cost? how much does each cup of coffee cost? i need this done

In september, louise planted a tree. every month, the tree louise planted grew 2 inches. what would the constant of proportionality of the line be?