Excellent Ticbol! I guess I never read the question carefully enough. I have one problem thoughusually, the general equation is written as y = A*cos B(x - C) + D. here C is the horizontal shift. as you have it in your equation, C/B is the horizontal shift.

it didn't affect the answer here though, since C = 0, but it might in some other question. (or i don't know, maybe in practice it won't make much of a difference, since the value of C may be altered by the way you set up the equation or something)

Good Job though!