Question

Melissa makes 9 dollars

Answer 1

Knowing she makes $9 for each 1 hour, you know that if you'd need to find how much she made after 4 hours, you would do 9(4), giving you the answer of $36.
With this knowledge, you can replace the 4 with h.

9(h)=P

The reason why it's such a simple equation is because it's per hour and nothing else is being increased, decreased, or changed.

Hope this helps!