3sin(a) + 3cos(a) = 1
3(sin(a) + cos(a)) = 1
sin(a) + cos(a) = 1/3
(sina + cosa)² = (1/3)²
sin²a +2sin(a)cos(a) + cos²a = 1/9
(sin²a + cos²a)+2sin(a)cos(a) = 1/9
1+2sin(a)cos(a) = 1/9
2sin(a)cos(a) = 1/9 - 1
2sin(a)cos(a) = sin(2a) = - 8/9
