How do I solve this?

Question

Find all points on the curve y = cos(x) where the tangent line is parallel to the line y = -x.

Answer 1

y = cosx

Slope of tangent = y' = -sinx

Slope of y = -x is -1

So, sinx = 1

x = pi / 2 + 2k(pi), k = any integer

Answer 2

At which values is cos x = -1 (Look at the cosine curve!!!)