How do you convert sin into cos, and vice versa ... Promblem: cos (6x) = sin (3x-9)

The rule for converting sin into cos and vice versa is:

sin α = cos (90° - α) or cos α = sin (90° - α)

In this case:

cos (6 x) = sin (90° - 6 x)

sin (90° - 6 x) = sin (3 x - 9°)

90° - 6 x = 3 x - 9°

90° + 9° = 3 x + 6 x

99° = 9 x

x = 99° : 9 = 11°

Answer: C) x = 11°