Math Help - forming a function

1. forming a function

Where do I start with this problem?

A store owner bought n dozen toy boats at a cost of $3 per dozen and sold them at$.75 a piece. Express the profit P(in dollars) as a function of n.

The answer is 6n.

Never mind, I figured it out.
I made the function of profit p=9n because I get $9 per dozen. Then I subtracted$9 minus the \$3 it takes to purchase the dozen.