What is the common difference between the elements of the arithmetic sequence below? -18, -22.5,-27,-31.5,-36

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?

Through non-collinear points there is exactly one plane.

Asap! i'm not good math and i'm lost.