Answer: Option B.
Explanation:
The given formula is

where,
a = actual cost of an item.
m = Original price
n = Amount of coupon
We need to solve the formula for the amount of the coupon (n).
In means, we need to isolate n in the given formula.
We have,

Subtract m form both sides.


Multiply both sides by -1.



So,
.
Therefore, the correct option is B.