3 - cos (3x) = 3cos (x)² + 3sin (x)²

3 - cos (3x) = 3 (cos (x)² + sin (x)²)

3-cos (3x) = 3×1

3 - cos (3x) = 3

-cos (3x) = 0

cos (3x) = 0

3x = /2 + k, k є Z

x = /6 + k/3, k є Z