It is known that 5sin β = sin (2 α + β), cos α ≠ 0, cos (α + β) ≠ 0

It is known that 5sin β = sin (2 α + β), cos α ≠ 0, cos (α + β) ≠ 0

5sinb=5sin(a+b-a)=5sin(a+b)cosa-5cos(a+b)sina
2sin(2a+b)=2sin(a+b+a)=2sin(a+b)cosa+2cos(a+b)sina
From the known conditions, we get 3cos (a + b) Sina = 2Sin (a + b) cosa. Now you should know