Answer:
15/17
Step-by-step explanation:
You can use what is called the Pythagorean identity:
sin(θ)² + cos(θ)² = 1
(-8/17)² + cos(θ)² = 1
cos(θ)² = 1 - 64/289 = 225/289
cos(θ) = √(225/289)
cos(θ) = 15/17 . . . . . . . cosine is positive in the 4th quadrant
