Dollar General sold garden hoses at a reduced price of $7.64 and took an end-of-season markdown of $12.35. What was the original selling price of each hose? Use the formula M S N  (Markdown = selling price- reduced price).

Well you have the formula, just rearrange it to give you the selling price then put the numbers in.

"Dollar General sold garden hoses at a reduced price of $7.64 and took an end-of-season markdown of $12.35. What was the original selling price of each hose? Use the formula M S N  (Markdown = selling price- reduced price)."

That formula gives you the answer in a round-about way. If the reduced price is $7.64 after a markdown of $12.35, then the original price was $19.99 (or S + M = OP). If you stuck with that formula then:
M = S - R
$12.35 = S - $7.64
$12.35 + $7.64 = S -$7.64 + $7.64
$19.99 = S