\(cos(4\theta) - cos(2\theta) = sin(\theta)\)
\(-2sin(\frac{4\theta + 2\theta}{2})sin(\frac{4\theta - 2\theta}{2}) = sin(\theta)\)
\(-2sin(\frac{6\theta}{2}sin\frac{2\theta}{2}) = sin(\theta)\)
\(-2sin(3\theta)sin(\theta) = sin(\theta)\)
\(\frac{-2sin(3\theta)sin(\theta)}{-2sin(\theta)} = \frac{sin(\theta)}{-2sin(\theta)}\)
\(sin(3\theta) = -\frac{1}{2}\)
\(sin^{-1}[sin(3\theta)] = sin^{-1}(-\frac{1}{2})\)
\(3\theta = -30\)
\(\frac{3\theta}{3} = \frac{-30}{3}\)
\(\theta = -10\)
