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!
