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.

Learn 2 /Fourier Transform/

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

>>2 >>3
Get back to kindergarten.

>>1
Remember that sin² + cos² = 1. [1]
Using the formula, you get:
A*sin(y) = -4
A*cos(y) = 3
Square and sum together:
A²(sin(y)² + cos(y)²) = (-4)² + 3² = 25
Use [1] to get A² = 25
=> A = ±5

So you have:
sin(y) = -4/5 or 4/5
cos(y) = 3/5 or -3/5
=> y = arctan(-4/3) + n*π