Hi there again, Cirular functions is my weakest concept in maths, could you please help me with this

The number of daylight hours, y in Melbourne, x days after the longest day of the year (December 22nd or December 21st) is moddelled approximately by the rule y=12.125+2.625cos [(2 pi x)/365]

a) for what values of x is the length of the day changing most rapidly
B) The model is based on days elapsed after the longest day. How can we change the formula to determine the daylight for days since the start of the year?

Any help would be appreciated

For part a, it would appear you can use the second derivative of y, then set to 0 and solve for x.

y=\frac{97}{8}+\frac{21}{8}cos(\frac{2{\pi}x}{365} )

y''=\frac{-21{\pi}^{2}cos(\frac{2{\pi}x}{365})}{266450}