a = your total price before the coupon and sales tax.
b = your coupon's given discount (for example, 25% off)
c = your discounted total
(a - (ab)) = c
c + (.075c) = your answer.
With some numbers plugged in:
a = 50$, b = -25%
50 - (50 x .25)
50 - 12.5 = 37.50$
37.5 + (37.5 x .075)
37.5 + 2.82* = 40.32$
So you would have to pay $40.32
