Answer:
Drawing out the quadrants cos of an angle is negative in the third quadrant. Your drawing should indicate that x and y are negative. Then since we are working with a right triangle, again your diagram should indicate that. Determine the value of the third side. You know one is - 6 and the other opposite the 90 degree angle, the hypotenuse is 10. cos of an angle is adjacent divided by the hypotenuse.
I hope this helps if it did not help use this
cosine is negative in Quadrant III.
Use a pythagorean theorem to find the other leg. a2 = c2 - b2
adjacent leg = √(102 - 82)
cos θ = -6/10