C3 questions involving trig addition formulae, a bit stuck :/

(ii) You know that y=3cosx + 12sinx = 13cos(x-67.4)
The same transformation rules apply as normal polynomial graphs- if something is inside the brackets, like the (x-67.4) part, do the opposite transformation to the x coordinates. If something is outside the brackets, like the 13 of the 13cos(x-67.4), do the transformation to the y co ordinates.

(iii) again use the fact that 13cos(x-67.4)=2
solve from there.

Ask if you need further explanation!

(i) Express 5Cosx + 12sinx in the form RCos(x-a), where R>0 and 0<a<90? I know that the answer for this is 13cos(x-67.4)

It's part two and three of the question that I'd like guided answers for, I dont really know where to start.

(ii) Hence give details of transformations which transforms the curve y=cosx to the curve y=5cosx + 12sinx

(iii) Solve, for 0<x<360, the equation 5cosx + 12sinx = 2, giving your answers correct to the nearest 0.1 degree.

thankyou people!!