Can someone please help me with this algebra

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

Jamie went to home depot. she bought 25 bags of soil that cost $9 per bag. she bought 15 pots at $8 each, and she bought 23 bags of pebbles at $15 each. she used a coupon that gave her $5 off for every 100 dollars she spent. how much did jame pay at the end?

What is the area of the original rectangle? 8 cm² 20 cm² 25 cm² 40 cm²