Answer
See below.
Work Step by Step
$LHS=sin(3\theta)=sin(2\theta+\theta)=sin(2\theta)cos\theta+cos(2\theta)sin\theta
=2sin\theta cos^2\theta+(1-2sin^2\theta)sin\theta
=2sin\theta (1-sin^2\theta)+(1-2sin^2\theta)sin\theta
=3sin\theta -4sin^3\theta
=RHS$
