F(4) for the function f(x)=2x+5

Can someone explain the elimination method to me for this problem?

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?

Christina can paint 420 square feet in 105 minutes. which unit rate describes the scenario?