Answer: 7
Explanation:
sin x = cos (90-x)
sin (3x + 7) = cos (90- (3x + 7))
simplifying the RHS
sin (3x + 7) = cos (83 - 3x)
therefore:
cos (83 - 3x) = cos (10x - 8)
eliminating the cos
83 - 3x = 10x - 8
adding 8 from both sides
83+ 8 -3x = 10x
91 - 3x = 10x
adding 3x to both sides
91 = 13x
dividing by 13
7 = x