Cosine, or cos, is the adjacent line/ the hypotenuse
In this case, the hypotenuse is 17 since it is the <u>longest side</u> in this right triangle. The adjacent line is 8 since it is adjacent, or <u>touching, to the Angle A</u>. 15 is the tangent that is <u>facing opposite from Angle A</u>.
If we plug in the given numbers correctly, the answer should be 8/17.