Suppose a certain variable star has a period of 10 days. The average brightness of the star is 3.9 magnitudes, and the maximum variation from the average is 0.4 magnitude. Assuming that the variation in brightness is simple harmonic, find an equation that gives the brightness of the star as a function of time. (Let y be the brightness in magnitudes and let t be the time in days. Suppose that when t = 0 the brightness is 3.4 magnitudes and increasing.)