I have the following equation $\displaystyle y= -4sin15x+25 $ ( y represents the temperature and x represents time in hours, so x=0 means midnight and x=1 means 1:00 am) and I am given a question with the graph of the given equation, and to get the solution to the question you can you use the graph (which is also what my teacher told us to do) and so i do use the graph and get the correct answer of 13 hours. My question is how would i use the equation to solve for what i need?

The question:

The pool is comfortable for swimming at 23 degrees Celsius. The pool is open from 6:00 am to 11:00 pm. How many hours of comfortable swimming are available on a sunny day?

Any help is appreciated and thank you in advance!