PART A:
and

If we apply the coupon first, we have
, because we are subtracting 5 dollars from the total price. Then, if we apply the discount, we get
. It is a 10% discount, which means the price will end up being 90% of the original price, so we need to multiply the price by 0.9. To get the second equation, I distributed the 0.9 (multiplied it by both parts of the equation in parentheses) to get
.
PART B:

If we apply the discount first, we get
. Then, when we apply the coupon, we subtract 5 dollars from that price, getting
.
PART C: Discount, then coupon.
To figure out which order will save more money, we look at the simplified equations for each order. In the first order, we end up subtracting 4.5 from
, and in the second order, we end up subtracting 5 from
. In the second order, we are subtracting more money, so the price will be lower and she will save more money.