(20,13)(40,26)
slope = (26 - 13) / (40 - 20) = 13/20
y = mx + b
slope(m) = 13/20
(20,13)...x = 20 and y = 13
now we sub and solve for b
13 = 13/20(20) + b
0 = b
equation is : y = 13/20x.....and when x = 280
y = 13/20(280) = 3640/20 = 182 <== cost of 280 baseball cards...but not the total cost.
7.7% of 182 = 0.077(182) = 14.01 <== the sales tax
so we have : 182 (cost of cards) + 14.01(sales tax) + 5.95(mailing fee) for a total cost of : $ 201.96 <== total cost of baseball cards