On which of the following intervals is the function f (x) = 4 cos (2x - π) decreasing?

We know on the regular cosine graph, it decreases from 0 to pi. So, we need to set 0 as y.

0 = 4 cos (2x-pi)

0 = cos (2x-pi)

2x-pi = pi

x = pi

We already know the answer, but do pi on your own!