Please help me understand where this expansion came from

[coverband]

Homework Statement

(asinz + bsin3z)^3 = (3a/4)(a^2 – ab + 2b^2)sinz – (1/4)(a^3 – 6a^2 – 3b^3)sin3z...

Homework Equations

The Attempt at a Solution

How is this so !? What is being used here (taylor/binomial...?)

 

[vela]

Nothing that exotic. It's just algebra and the application of trig identities to get everything in terms of just sin z and sin 3z.