I'm a tutoring a girl and I need this.

I'd appreciate if someone could help.

using the identity sin(a+b) = sin(a)cos(b) + cos(a)sin(b)

show -4cos(3x) + 3sin(3x) in the form Asin(3x+y)

I'm screwed.

you're fucking retarded.
just use the formula.
chose a value of a so that
sin(a) = -4/3 and cos(a) = 1
then it'll be easy