The formula comes from asinφ +/- bcosφ = Rsin(φ+α)
It has just been expanded using the addition formulas - these are a couple of pages back if you have the same book I do
You'll usually be told which to use .. Give your answer in the form Rsin.. And Rcos...
If not, just go with whichever comes first
For example with yours, use the sin formula as sin is first in the equation
I'm doin C4 next week and don't remember ever getting a question where you had to use the R addition formulas without being told to or how to leave your answer