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°