Answer:
We know that the expression that models the situation is:
45 - n
Where n is the amount that she lost.
We know that she lost $3 for each hour closed.
And the shop was 8 hours closed, so the total loss is:
8*$3 = $24
The profit is the difference between the revenue (we know that it is $45) and the loss (we know that it is $24)
Then the profit is:
$45 - $24 = $21