Since A is the amplitude, this part is easy: A = 3.
Now, we want a 0 of the sine function to be positioned at . We can do this by setting at , so we have the equation:
Again, we want a 0 of the sine function to be positioned at . We can do this by setting ( is the next zero for the sine function after 0) at , so we have the equation:
Solving the first equation for gives:
Inserting this into the second equation gives:
( is a convenient first zero for cos)
(the next zero for cos after )
The other way is to recall that so to switch to cosine we need to "shave" a off the argument.
Either way I got: