To solve this problem, we will use the given supply function for scarves, which is [tex]\( P = Q - 9 \)[/tex], where [tex]\( P \)[/tex] is the price of the scarves and [tex]\( Q \)[/tex] is the quantity of scarves supplied.
We are told that Sandy sells the scarves for [tex]\( \$12 \)[/tex] each, so we substitute [tex]\( P = 12 \)[/tex] into the supply function:
[tex]\[ 12 = Q - 9 \][/tex]
Next, we solve for [tex]\( Q \)[/tex] by isolating [tex]\( Q \)[/tex] on one side of the equation. Add 9 to both sides of the equation:
[tex]\[ 12 + 9 = Q \][/tex]
[tex]\[ 21 = Q \][/tex]
Therefore, the store will supply [tex]\( 21 \)[/tex] scarves when they are sold for [tex]\( \$ 12 \)[/tex] apiece.
The correct answer is:
D. 21 scarves
