Question

this is homework, i just need help on how to get started, i do not need an answer given to me just the method!
13) a) express 12cosx+9sinx in the form Rcos(x-α) where R>0 0< Θ<π/2

i did tanα=b/a
α=0.644 radians

and R=√a²+b²
R=15

so i now have

15cos(x-0.644)


B) use the method of part (a) to find the smallest positive root α of the equation

12cosx+9sinx=14 giving your answer correct to three decimal places


NOTE: I have done part a, i just need help with part b