Justin is a very fluctuant employee. He sells the max of 200 dollars worth of product on day 3. He sells the min of 30 dollars worth of product on day 7. Let $t=0$ represent the day he started working this job. Write the equation that represents the sales amount as a function of time.
I got $ y=-85\cos((2\pi/7)x)+115$.
My reasoning is that $115-85=30$ (the min) and $115+85 = 200 $ (the max). Is my reasoning in the right direction?
Yes, I do believe your reasoning is looking in the correct direction. However, I can say that your equation is incorrect, because it reaches the max at day $3.5$ instead of day $3$. I would recommend next steps involving adjusting how you dilate and translate horizontally in order to fix this misalignment.