(10,84)(20,159)
slope(m) = (159 - 84) / (20 - 10) = 75/10 = 7.5
y = mx + b
slope(m) = 7.5
use either of ur points...(10,84)...x = 10 and y = 84
now we sub and solve for b, the y int
84 = 7.5(10) + b
84 = 75 + b
84 - 75 = b
9 = b
so ur equation is : y = 7.5x + 9...with each shirt costing $ 7.50 and the one-time set up fee costing $ 9.00...with y being the total cost and x being the number of shirts bought
for 30 shirts......x = 30
y = 7.5(30) + 9
y = 225 + 9
y = 234 <== cost of an order of 30 shirts
