Current in circuit (trig problem)

Hey this is my first post... been really stuck on this particular question from a paper:

In a particular circuit the current, I amperes, is given by:


I=4sinθ - 3cosθ θ>0


where θ is an angle related to the voltage.


Given that I=R x sin(θ-a), where R>0 and 0<=a<360degrees,


a) find the value of R, and the value of a to 1 decimal place.
b) Hence solve the equation 4sinθ - 3cosθ = 3 to find the values of θ between 0 and 360degrees.
c) Write down the greatest value for I.
d) Find the value of θ between 0 and 360degrees at which the greatest value of I occurs.

I think if someone could show me the steps to answering part a) I could be able to do the rest myself.
Thanks