Find cos(a+b) if sin(a)= -8/17 where a is in the fourth quadrant and tan(b)= -20/21 where b is in the second quadrant.?

sina = -8/17
cos²a = 1 - (-8/17)² = 225/289
cosa = 15/17 for a is in the Ⅳ quadrant.

tanb = -20/21
(sin²b) / cos²b = (-20/21)²
(sin²b) / (1 - sin²b) = 400/441
400 - 400sin²b = 441sin²b
sin²b = 400/841
sinb = 20/29 for b is in the Ⅱ quadrant.
cos²b = 1 - 400/841
cosb = -21/29 for b is in the Ⅱquadrant.

cos(a+b) = cosa cosb - sina sinb = (15/17)(-21/29) - (-8/17)(20/29) = -155/493. ANSWER : A)